LC 102. Binary Tree Level Order Traversal

1.题目描述

102. Binary Tree Level Order Traversal

Medium

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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],

    3
   / \
  9  20
    /  \
   15   7

 

return its level order traversal as:

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

2.解题思路

用队列实现对二叉树的层次遍历

3.实现代码

/**
 * 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> levelOrder(TreeNode* root) {
        queue Q;
        vector> V;
        if (root==NULL)
            return V;
        Q.push(root);
        while (!Q.empty()) {
            int n = Q.size();
            vector v;
            for (int i=0; ileft) Q.push(Q.front()->left);
                if (Q.front()->right) Q.push(Q.front()->right);
                v.push_back(Q.front()->val);
                Q.pop();
            }
            V.push_back(v);
        }
        return V;
    }
};