leetcode 102. Binary Tree Level Order Traversal

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#leetcode #102 #Binary Tree Level Or

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本文介绍了一种解决二叉树层次遍历问题的方法。通过使用队列来辅助实现从上到下、从左到右的层次遍历，并返回每一层节点的值。示例中给出了一棵二叉树的具体层次遍历过程。

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

/**
 * 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) 
    {
        vector<vector<int>> ret;
        if (!root)
            return ret;
            
        queue<TreeNode*> que;
        que.push(root);
        
        while (!que.empty())
        {
            int size = que.size();
            vector<int> temp;
            for (int i = 0; i < size; i++)
            {
                TreeNode *p = que.front();
                que.pop();
                temp.push_back(p->val);
                if (p->left)
                    que.push(p->left);
                if (p->right)
                    que.push(p->right);
            }
            ret.push_back(temp);
        }
        return ret;
    }
};