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