865. Smallest Subtree with all the Deepest Nodes.cpp

Given the root of a binary tree, the depth of each node is the shortest distance to the root.

Return the smallest subtree such that it contains all the deepest nodes in the original tree.

A node is called the deepest if it has the largest depth possible among any node in the entire tree.

The subtree of a node is tree consisting of that node, plus the set of all descendants of that node.

Note: This question is the same as 1123: https://leetcode.com/problems/lowest-common-ancestor-of-deepest-leaves/



Example 1:


Input: root = [3,5,1,6,2,0,8,null,null,7,4]
Output: [2,7,4]
Explanation: We return the node with value 2, colored in yellow in the diagram.
The nodes coloured in blue are the deepest nodes of the tree.
Notice that nodes 5, 3 and 2 contain the deepest nodes in the tree but node 2 is the smallest subtree among them, so we return it.
Example 2:

Input: root = [1]
Output: [1]
Explanation: The root is the deepest node in the tree.
Example 3:

Input: root = [0,1,3,null,2]
Output: [2]
Explanation: The deepest node in the tree is 2, the valid subtrees are the subtrees of nodes 2, 1 and 0 but the subtree of node 2 is the smallest.


Constraints:

The number of nodes in the tree will be in the range [1, 500].
0 <= Node.val <= 500
The values of the nodes in the tree are unique.




/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    unordered_map<TreeNode*, int> mp;
    int max_depth;

    void dfs(TreeNode* root, TreeNode* parent){
        if(root){
            mp.insert({root, mp[parent]+1});
            dfs(root->left, root);
            dfs(root->right, root);
        }
    }

    TreeNode* answer(TreeNode* root){
        if(root==NULL || mp[root]==max_depth) return root;
        TreeNode* L=answer(root->left);
        TreeNode* R=answer(root->right);
        if(L && R) return root;
        if(L) return L;
        if(R) return R;
        return NULL;
    }

    TreeNode* subtreeWithAllDeepest(TreeNode* root) {
        mp.insert({NULL, -1});
        dfs(root, NULL);
        max_depth=-1;
        for(auto it=mp.begin();it!=mp.end();it++){
            max_depth=max(max_depth, it->second);
        }
        return answer(root);
    }
};