leetcode515+每一层tree的节点最大值,BFS

https://blog.youkuaiyun.com/a2331046/article/details/70149011

/**
 * 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) {
        queue<TreeNode*> Q;
        vector<int> res;
        if(!root) return res;
        Q.push(root);
        while (!Q.empty()) {
            int size = Q.size();
            int max = INT_MIN;
            while (size--) { //记录了每一层的个数
                TreeNode* node = Q.front();
                Q.pop();
                if(node->val > max) max = node->val;
                if(node->left) Q.push(node->left);
                if(node->right) Q.push(node->right);
            }
            res.push_back(max);
        }
        return res;
    }
};