Last Node in a Complete Binary Tree

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
void dfs(TreeNode* A, int dep, pair<int,int>& res) {
    if(!A) return;
    if(res.first < dep) res = {dep,A->val};
    else if(res.first == dep) res.second = A->val;
    dfs(A->left, dep + 1, res);
    dfs(A->right, dep + 1, res);
}
int Solution::lastNode(TreeNode* A) {
    pair<int, int> res{-1,-1};
    dfs(A, 0, res);
    return res.second;
}