19.3.4 [LeetCode 102] 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).

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]
]

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        queue<TreeNode*>q, p;
        vector<vector<int>>ans;
        if(!root)return ans;
        int i = 0;
        q.push(root);
        while (!q.empty()) {
            ans.push_back(vector<int>());
            while (!q.empty()) {
                TreeNode*now = q.front(); q.pop();
                ans[i].push_back(now->val);
                if (now->left)
                    p.push(now->left);
                if (now->right)
                    p.push(now->right);
            }
            q = p;
            p = queue<TreeNode*>();
            i++;
        }
        return ans;
    }
};