865. Smallest Subtree with all the Deepest Nodes

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

c++

/**
 * 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:
    TreeNode* subtreeWithAllDeepest(TreeNode* root) {
        if(!root) return root;
        if(!root->right and !root->left) return root;
        unordered_map<TreeNode*, TreeNode*> parentMap;
        queue<TreeNode*> q;
        unordered_set<TreeNode*> deepNodes{root};
        q.push(root);
        while(!q.empty()) {
            int sz = q.size();
            unordered_set<TreeNode*> temp;
            while(sz--) {
                auto n = q.front();
                q.pop();
                if(n->left) {
                    parentMap[n->left] = n;
                    temp.insert(n->left);
                    q.push(n->left);
                }
                if(n->right) {
                    parentMap[n->right] = n;
                    temp.insert(n->right);
                    q.push(n->right);
                }
            }
            if(!temp.empty()) swap(deepNodes, temp);
        }
        
        while(deepNodes.size() != 1) {
            unordered_set<TreeNode*> parentNode;
            for(auto n : deepNodes) {
                parentNode.insert(parentMap[n]);
            }
            swap(parentNode, deepNodes);
        }

        return *deepNodes.begin();
    }
};

c++

/**
 * 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 {
    pair<int, TreeNode*> findDeep(TreeNode* node, int depth) {
        if(!node) return {0, node};
        auto l = findDeep(node->left, depth + 1), r = findDeep(node->right, depth + 1);
        return {max(l.first, r.first) + 1, l.first == r.first ? node : l.first > r.first ? l.second : r.second};
    }
public:
    TreeNode* subtreeWithAllDeepest(TreeNode* root) {
        return findDeep(root, 0).second;
    }
};