102. Binary Tree Level Order Traversal

problem description

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]
]

algorithm thought

二叉树层次遍历，使用queue辅助解决，必须掌握的题目

code

/**
 * 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>> res;
        TreeNode* pos=new TreeNode(-100);
        vector<int> tmp;
        queue<TreeNode*> q;
        q.push(root);
        q.push(pos);
        while(!q.empty()){
            TreeNode* tq=q.front();
            q.pop();
            if(tq==NULL){
                continue;   
            }
            else if(tq==pos){
                if(q.empty())
                    break;
                res.push_back(tmp);
                tmp.clear();
                q.push(pos);
            }
            else{
                q.push(tq->left);
                q.push(tq->right);
                tmp.push_back(tq->val);
            }
        }
        return res;
    }
};

algorithm analysis

遍历一棵树，时间复杂度O(n)