leetcode NO.102二叉树的层序遍历

利用队列的特性把每一层的节点出队并把下一层的节点入队即可，注意每一层的循环条件size要提前保存不能直接使用queue.size()因为队列的大小在变化因此无法判断每一层的出队情况

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        queue<TreeNode*> w;
        if (root)
            w.push(root);
        while (!w.empty()) {
            vector<int> temp;
            TreeNode* node;
            int size = w.size();
            for (int i = 0; i < size; i++) {
                node = w.front();
                temp.push_back(node->val);
                if (node->left)
                    w.push(node->left);
                if (node->right)
                    w.push(node->right);
                w.pop();
            }
            result.push_back(temp);
        }
        return result;
    }
};