added more annotations and trailing newline

Author: rustykitty

0009-palindrome-number.cpp

@@ -12,8 +12,8 @@ class Solution {
     bool isPalindrome(int x) {
         if (x < 0) return false;
         // Reverse digits
-        long y = 0;
-        int x2 = x;
+        long y = 0; // we have to use a long because some inputs 
+        int x2 = x; // exceed 2^31-1 when reversed
         while (x2 != 0) {
             y *= 10;
             y += x2 % 10;

0014-longest-common-prefix.c

@@ -22,7 +22,7 @@ char* longestCommonPrefix(char** strs, int strsSize) {
     for (unsigned char i = 0; i < len_shortest; i++) {
         for (char** p = strs; p != strsEnd; ++p ) {
             char* str = *p;
-            // this means that the prefix ends here
+            // not equal, prefix ends here
             if (str[i] != strs[0][i]) return prefix;
         }
         prefix[i] = strs[0][i];

0014-longest-common-prefix.cpp

@@ -20,7 +20,7 @@ class Solution {
         }
         for (unsigned char i = 0; i < len_shortest; i++) {
             for ( const auto& str : strs ) {
-                // this means that the prefix ends here
+                // not equal, prefix ends here
                 if ( str[i] != strs.front()[i] ) return prefix;
             }
             prefix += strs.front()[i];

0020-valid-parentheses.cpp

@@ -15,16 +15,16 @@ class Solution {
         for (const char& c : s) {
             if (c == '(' || c == '[' || c == '{') { // opening parentheses
                 stack.push_back(c);
-            } else if (stack.empty() ||
+            } else if (stack.empty() || // back() when deque is empty is UB
                        stack.back() != (c == ')' ? '(' : // matching closing
                                         c == ']' ? '[' : // parentheses with
                                         c == '}' ? '{' : // open parentheses
                                                    c)) {
                 return false;
-            } else {
+            } else { // valid pair
                 stack.pop_back();
             }
         }
-        return stack.empty();
+        return stack.empty(); 
     }
 };

0021-merge-two-sorted-lists.c

@@ -34,4 +34,4 @@ struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2) {
     }
     tail->next = list1 == NULL ? list2 : list1;
     return head;
-}
+}

0021-merge-two-sorted-lists.cpp

@@ -23,11 +23,14 @@ class Solution {
     ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
         if (list1 == nullptr) return list2;
         if (list2 == nullptr) return list1;
+        // start with whichever value is greater
         ListNode* head = list1->val > list2->val ? list2 : list1;
+        // advance that list to the next element
         if (head == list1) list1 = list1->next;
         else list2 = list2->next;
         ListNode* tail = head;
         while (list1 != nullptr && list2 != nullptr) {
+            // find smaller value and advance list
             if (list1->val > list2->val) {
                 tail->next = list2;
                 list2 = list2->next;
@@ -37,7 +40,8 @@ class Solution {
             }
             tail = tail->next;
         }
+        // everything else is tacked on to the end
         tail->next = list1 == nullptr ? list2 : list1;
         return head;
     }
-};
+};

0028-find-the-index-of-the-first-occurrence-in-a-string.cpp

@@ -17,4 +17,4 @@ class Solution {
         }
         return -1;
     }
-};
+};

0058-length-of-last-word.py

@@ -9,4 +9,4 @@
 
 class Solution:
     def lengthOfLastWord(self, s: str) -> int:
-        return len(s.strip().split()[-1]) # split into words and get last element in list
+        return len(s.strip().split()[-1]) # split into words and get last element in list

0083-remove-duplicates-from-sorted-list.cpp

@@ -32,4 +32,4 @@ class Solution {
         }
         return head;
     }
-};
+};

0125-valid-palindrome.py

@@ -9,6 +9,6 @@
 
 class Solution:
     def isPalindrome(self, s: str) -> bool:
-        # convert the input string to lowercase, then remove all non-alphanumeric
-        s = ''.join(c for c in s.lower() if c.isalnum()) # characters
-        return s == s[::-1] # whether the resulting string is equal to itself reversed
+        # convert the input string to lowercase, then remove all non-alphanumeric chars
+        s = ''.join(c for c in s.lower() if c.isalnum())
+        return s == s[::-1] # whether the resulting string is equal to itself reversed

0141-linked-list-cycle.cpp

@@ -28,4 +28,4 @@ class Solution {
         }
         return false;
     }
-};
+};

0144-binary-tree-preorder-traversal.py

@@ -23,4 +23,4 @@ def t(root):
             t(root.left)
             t(root.right)
         t(root)
-        return b
+        return b

0168-excel-sheet-column-title.cpp

@@ -18,4 +18,4 @@ class Solution {
         }
         return col;
     }
-};
+};

0171-excel-sheet-column-number.cpp

@@ -6,7 +6,7 @@ Submitted: November 27, 2024
 Runtime: 0 ms (beats 100.00%)
 Memory: 8.77 MB (beats 31.86%)
 
-See 0171-excel-sheet-column-number-alternative.cpp for an alternative solution.
+See `0171-excel-sheet-column-number-alternative.cpp` for an alternative solution.
 */
 
 class Solution {
@@ -15,8 +15,8 @@ class Solution {
         int res = 0;
         for ( const char& c : columnTitle ) {
             res *= 26;
-            res += (int) c - 64;
+            res += (int) c - 64; // 'A' == 65
         }
         return res;
     }
-};
+};

0175-combine-two-tables.sql

@@ -9,5 +9,5 @@ Runtime: 399 ms (beats 83.54%)
 # Write your MySQL query statement below
 SELECT firstName, lastName, city, state
 FROM Person
-LEFT JOIN Address -- we need to use LEFT JOIN so that `null` is produced if the
+LEFT JOIN Address -- LEFT JOIN means that `null` is produced if the
 USING (personId); -- address of a Person is not found in Address

0190-reverse-bits.cpp

@@ -17,4 +17,4 @@ class Solution {
         }
         return result;
     }
-};
+};

0191-number-of-1-bits.cpp

@@ -18,4 +18,4 @@ class Solution {
         }
         return res;
     }
-};
+};

0195-tenth-line.sh

@@ -1,11 +1,9 @@
-: <<'###'
-195. Tenth Line
+# 195. Tenth Line
 
-Submitted: October 31, 2024
+# Submitted: October 31, 2024
 
-Runtime: 19 ms (beats 92.92%)
-Memory: 3.85 MB (beats 41.81%)
-###
+# Runtime: 19 ms (beats 92.92%)
+# Memory: 3.85 MB (beats 41.81%)
 
 # Read from the file file.txt and output the tenth line to stdout.
-head -n 10 file.txt | tail -n +10
+head -n 10 file.txt | tail -n +10

0202-happy-number.cpp

@@ -12,7 +12,7 @@ class Solution {
     bool isHappy(int n) {
         int fast = sumDigits(sumDigits(n));
         int slow = sumDigits(n);
-        while (fast != slow) { // Floyd's cycle detection algorithim
+        while (fast != slow) { // cycle detection algorithim
             fast = sumDigits(sumDigits(fast));
             slow = sumDigits(slow);
         }

0206-reverse-linked-list.cpp

@@ -28,7 +28,7 @@ class Solution {
             head->next = head_next->next;
             head_next->next = new_head;
             new_head = head_next;
-        } // remove node after old "head" and add to front
+        } // remove node after original head and add to front
         return new_head;
     }
 };

0258-add-digits.cpp

@@ -20,4 +20,4 @@ class Solution {
         }
         return num;
     }
-};
+};

0278-first-bad-version.cpp

@@ -1,6 +1,8 @@
 /*
 278. First Bad Version
 
+Submitted: November 5, 2024
+
 Runtime: 1301 ms (beats 5.33%)
 Memory: 7.58 MB (beats 62.02%)
 */
@@ -13,7 +15,8 @@ Memory: 7.58 MB (beats 62.02%)
 class Solution {
 public:
     int firstBadVersion(int n) {
-        while(isBadVersion(n)) n--;
-        return n + 1;
+        
+        while (isBadVersion(n)) n--; // go back by one until last good ver
+        return n + 1; // ver after last good ver is first bad one
     }
-};
+};

0367-valid-perfect-square.cpp

@@ -9,10 +9,10 @@ class Solution {
 public:
     bool isPerfectSquare(int num) {
         if (num == 1) return true;
-        int max = num < 46341 ? num : 46341; // 46341^2 is over 2^31-1,
-        for (int i = 0; i < max; ++i) {      // the max value of a 32-bit
-            if (i * i == num) return true;   // unsigned integer
+        int max = num < 46341 ? num : 46341; // 46341^2 > 2^31-1 (INT_MAX)
+        for (int i = 0; i < max; ++i) {
+            if (i * i == num) return true;
         }
         return false;
     }
-};
+};

0476-number-compliment.cpp

@@ -15,9 +15,9 @@ class Solution {
         if (n == 0) return 1;
         int digits = 0;
         while (n != 0) {
-            n >>= 1;       // digits has the same number of `1` bits as
-            digits <<= 1;  // the number of digits total in `n` when you
-            digits++;      // remove leading zeroes
+            n >>= 1;      // number of 1 bits in digits is the
+            digits <<= 1; // total number of digits in n without leading zeroes
+            digits++;
         }
         return d ^ digits; // 1 ^ 1 = 0, 0 ^ 1 = 1
     }

0482-license-key-formatting.cpp

@@ -12,9 +12,9 @@ class Solution {
     string licenseKeyFormatting(string s, int k) {
         unsigned short i = 0;
         string result;
-        // we go through the string backwards so that the first group can
-        // contain less than `k` digits. it's easier to make the last group
-        // different than the first group different.
+        // go through the string backwards so that the first group can
+        // contain less than `k` digits; easier to make the last group
+        // different than the first group different
         for (auto it = s.crbegin(); it != s.crend(); ++it) {
             const char& c = *it;
             if (c == '-') { // ignore hyphens

0509-fibonacci-number.cpp

@@ -12,4 +12,4 @@ class Solution {
     int fib(int n) {
         return n < 2 ? n : (fib(n - 1) + fib(n - 2));
     }
-};
+};

0509-fibonacci-number.py

@@ -11,4 +11,4 @@ class Solution:
     def fib(self, n: int) -> int:
         if n == 0 or n == 1:
             return n
-        return self.fib(n - 1) + self.fib(n - 2)
+        return self.fib(n - 1) + self.fib(n - 2)

0520-detect-capital.cpp

@@ -29,4 +29,4 @@ class Solution {
         }
         return true;
     }
-};
+};

0535-encode-and-decode-tinyurl.java

@@ -27,4 +27,4 @@ public String decode(String shortUrl) {
 
 // Your Codec object will be instantiated and called as such:
 // Codec codec = new Codec();
-// codec.decode(codec.encode(url));
+// codec.decode(codec.encode(url));

0700-search-in-a-binary-search-tree.cpp

@@ -31,4 +31,4 @@ class Solution {
             return root->val == val ? root : nullptr;
         }
     }
-};
+};

0709-to-lower-case.c

@@ -12,4 +12,4 @@ char* toLowerCase(char* s) {
         if ('A' <= *p && *p <= 'Z') *p = *p + 32;
     }
     return s;
-}
+}

0709-to-lower-case.cpp

@@ -11,8 +11,8 @@ class Solution {
 public:
     string toLowerCase(string s) {
         for (char& c : s) {
-            if ('A' <= c && c <= 'Z') c = c + 32;
+            if ('A' <= c && c <= 'Z') c = c + 32; // 32 = 'a' - 'A'
         }
         return s;
     }
-};
+};

0709-to-lower-case.py

@@ -10,4 +10,4 @@
 class Solution:
     def __init__(self):
         # this is a literal joke
-        self.toLowerCase = str.lower
+        self.toLowerCase = str.lower

0771-jewels-and-stones.cpp

@@ -20,4 +20,4 @@ class Solution {
         }
         return res;
     }
-};
+};

0876-middle-of-the-linked-list.cpp

@@ -28,4 +28,4 @@ class Solution {
         }
         return slow;
     }
-};
+};

0977-squares-of-a-sorted-array.cpp

@@ -16,4 +16,4 @@ class Solution {
         std::sort(nums.begin(), nums.end());
         return nums;
     }
-};
+};

1009-complement-of-base-10-integer.cpp

@@ -1 +1 @@
-// See `0476-number-compliment.cpp`
+// See `0476-number-compliment.cpp`.

1108-defanging-an-ip-address.py

@@ -9,4 +9,4 @@
 
 class Solution:
     def defangIPaddr(self, address: str) -> str:
-        return address.replace('.', '[.]')
+        return address.replace('.', '[.]')

1290-convert-binary-number-in-a-linked-list-to-integer.cpp

@@ -28,4 +28,4 @@ class Solution {
         }
         return res;
     }
-};
+};

1313-decompress-run-length-encoded-list.c

@@ -18,7 +18,7 @@ int* decompressRLElist(int* nums, int numsSize, int* returnSize) {
         *returnSize += *(p);
         p += 2;
     }
-    int* returnArray = malloc(*returnSize << 2);
+    int* returnArray = malloc(*returnSize << 2); // 32 bits = 4 bytes
     p = nums;
     int* r = returnArray;
     while (p != end) {

1313-decompress-run-length-encoded-list.cpp

@@ -19,4 +19,4 @@ class Solution {
         }
         return res;
     }
-};
+};