102. Binary Tree Level Order Traversal二叉树层序遍历

网址：https://leetcode.com/problems/binary-tree-level-order-traversal/

参考：https://www.cnblogs.com/grandyang/p/4051321.html

二叉树的层次遍历的做法是维护一个队列，不断使节点入队，使用了val后即出队。结束标志是队列是否为空

主要难度在于要分别提取出每层的val

我们可把在树的一层节点全部入列后队列的长度作为for循环的判断依据

/**
 * 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<vector<int>> levelOrder(TreeNode* root) {
        if (!root) 
            return {};
        vector<vector<int>> res;
        queue<TreeNode*> q;
        q.push(root);
        vector<int> oneLevel;
        while (!q.empty()) {
            oneLevel.clear();
            // 一轮for循环即是对一层节点的处理
            for (int i = q.size(); i > 0; --i) {
                TreeNode *t = q.front();
                q.pop();
                oneLevel.push_back(t->val);
                if (t->left)
                    q.push(t->left);
                if (t->right)
                    q.push(t->right);
            }
            res.push_back(oneLevel);
        }
        return res;
    }
};