Leetcode102

Binary Tree Level Order Traversal:
Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

Solution:

#include <iostream>
#include <vector>
#include <queue>

using namespace std;

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>> res;
        if( root == NULL )
            return res;

        queue< pair<TreeNode*, int> > q;
        q.push( make_pair(root, 0));
        while ( !q.empty() ){

            TreeNode* node = q.front().first;
            int level = q.front().second;
            q.pop();

            if( level == res.size() ) //注意level是从0开始，是否需要增加新的一层
                res.push_back( vector<int>() );

             res[level].push_back( node -> val );

             if( node -> left)
                 q.push( make_pair( node ->left, level + 1));
             if( node -> right)
                 q.push( make_pair( node ->right, level + 1));
        }

        return res;

    }
};

总结： 队列最基本的应用–树的广度优先搜索。最基本的题一定要掌握。