leetcode -- 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]

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/find-largest-value-in-each-tree-row
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我的思路：

1. 用广搜，但是这道题在leetcode上是深搜，我感觉用广搜好解一点，于是选择广搜

2. 因为要记录每一层值，所以在每次进入while循环，先看里面有几个元素，就是每一层的结点个数

3. 然后再在每层中找最大值即可

/**
 * 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:
    queue<TreeNode*> q; //存放值
    vector<int> maxValue;
    vector<int> largestValues(TreeNode* root) {
        if(root==NULL) return maxValue;
        q.push(root);
        while(!q.empty()){
            int size = q.size();
            int maxnV = INT_MIN;
            for(int i = 0; i < size; i++){
                TreeNode* front = q.front();
                int v = front->val; //队头的值
                maxnV = max(maxnV, v);
                q.pop();
                if(front->left) q.push(front->left);
                if(front->right) q.push(front->right);
            }
            maxValue.push_back(maxnV);                 
        }
        return maxValue;
    }
};