第78题 Binary Tree Level Order Traversal

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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，并提供了一个C++实现示例。该算法采用广度优先搜索策略，按从左到右、逐层的顺序返回节点值。

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,#,#,15,7},

return its level order traversal as:

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

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Solution in C++:

/**
 * 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>> result;
        if(root==NULL) return result;
        queue<TreeNode*> q;
        q.push(root);
        int numChildren = 1;
        while(!q.empty()){
            int index = numChildren;
            numChildren =0;
            vector<int> curLevel;
            while(index>0){
                TreeNode* cur = q.front();
                q.pop();
                if(cur->left!=NULL){
                    q.push(cur->left);
                    numChildren++;
                }
                if(cur->right!=NULL){
                    q.push(cur->right);
                    numChildren++;
                }
                index--;
                curLevel.push_back(cur->val);
            }
            result.push_back(curLevel);
        }
        return result;
    }
};