17.Find Left Most Element

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int findLeftMostNode(TreeNode* root) {
        pair<int, int> rst = findLeftMostNode(root, 0);
        return rst.second;
    }
    
    pair<int, int> findLeftMostNode(TreeNode* root, int level) {
        if (!root) {
            return {-1, 0};
        }
        if (!root->left && !root->right) {
            return {level, root->val};
        }
        
        pair<int, int> l = findLeftMostNode(root->left, level + 1);
        pair<int, int> r = findLeftMostNode(root->right, level + 1);
        
        return l.first >= r.first ? l : r;
    }
};