515. Find Largest Value in Each Tree Row ----------M

 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]

A:

layer wise traversal

/**
 * 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:
    vector<int> largestValues(TreeNode* root) {
        vector<int> res;
        vector<TreeNode * > curLayer;        
        if(root)
            curLayer.push_back(root);
        while( not curLayer.empty()){
            vector<TreeNode * > nextLayer;
            int maxVal = INT_MIN;
            for(auto v : curLayer){
                maxVal = max(maxVal, v->val );
                if(v->left){
                    nextLayer.push_back(v->left);
                }
                if(v->right){
                    nextLayer.push_back(v->right);
                }
            }
            res.push_back(maxVal);
            curLayer = nextLayer; // can further use a pointer to avoid copy
        }
        return res;
    }
};

错误：

一开始没有检查root 是不是Null