515. Find Largest Value in Each Tree Row

问题描述：

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]

解题思路：

由于是计算每层的最大值，故可以在遍历该层次的时候去比较大小。

class Solution {
public:
    vector<int> largestValues(TreeNode* root) {
        vector<int> sum;
        if(root==NULL) return sum;
        sum.push_back(root->val);
        dfs(root->left,1,sum);
        dfs(root->right,1,sum);
        return sum;
    }
   void dfs(TreeNode* root,int level,vector<int> &nums)
    {
        if(root==NULL) return;
        else if(nums.size()<level+1)
            nums.push_back(root->val);
        else nums[level]=max(nums[level],root->val);
        dfs(root->left,level+1,nums);
        dfs(root->right,level+1,nums);
       
    }
};