1123. Lowest Common Ancestor of Deepest Leaves

/**
 * 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* lcaDeepestLeaves(TreeNode* root)
    {
        if (!root) return nullptr;

        int left = getDepth(root->left);
        int right = getDepth(root->right);

        if (left == right) return root;
        return (left > right) ? lcaDeepestLeaves(root->left) : lcaDeepestLeaves(root->right);
    }

    int getDepth(TreeNode* node)
    {
        if (!node) return 0;
        return 1 + max(getDepth(node->left), getDepth(node->right));
    }
};

• T: $O(N^2)$
• S: $O(H)$

Optimization

/**
 * 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* lcaDeepestLeaves(TreeNode* root)
    {
        return dfs(root).first;
    }
    pair<TreeNode*, int> dfs(TreeNode* node)
    {
        if (!node) return {nullptr, 0};

        auto left = dfs(node->left);
        auto right = dfs(node->right);

        if (left.second > right.second) return {left.first, left.second + 1};
        if (left.second < right.second) return {right.first, right.second + 1};
        return {node, left.second + 1};
    }
};

• T: $O(N)$
• S: $O(H)$