515. Find Largest Value in Each Tree Row

You need to find the largest value in each row of a binary tree.

Example:
Input:

      1
     / \
    3   2
   / \   \  
  5   3   9 

Output: [1, 3, 9]

方法1: dfs

思路：

/**
 * 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:
    vector<int> largestValues(TreeNode* root) {
        vector<int> res;
        dfs(root, 0, res);
        return res;
    }
    
    void dfs(TreeNode* root, int level, vector<int> & res) {
        if (!root) return;
        if (level == res.size()) res.push_back({INT_MIN});
        res[level] = max(res[level], root -> val);
        dfs(root -> left, level + 1, res);
        dfs(root -> right, level + 1, res);
        return;
    }
};