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]
]

/**
 * 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;
        queue<TreeNode *> que;

        if(root != NULL){
            que.push(root);

            while(!que.empty()){
                int len = que.size();
                vector<int> level;
                while(len-- > 0){
                    root = que.front();
                    que.pop();
                    level.push_back(root->val);

                    if(root->left) que.push(root->left);
                    if(root->right) que.push(root->right);
                }
                res.push_back(level);
            }
        }

        return res;
    }
};

Binary Tree Level Order Traversal II

将上面的结果reverse