(BFS 二叉树) 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]

--------------------------------------------------------------------------
就是找出二叉树的每一层的最大数。用BFS较为简单。

C++代码：

/**
 * 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> vec;
        if(!root){
            return vec;
        }
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            int res = -2147483648;  //int类型的最小数。
            for(int i = q.size(); i > 0; i--){
                auto t = q.front();
                q.pop();
                if(t->val > res){
                    res = t->val;
                }
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);
            }
            vec.push_back(res);
        }
        return vec;
    }
};