题目

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]
]

频度： 4

解题之法

/**
 * 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>> ret;
      buildVector(root, 0, ret);
      return ret;
}

void buildVector(TreeNode *root, int depth, vector<vector<int> >& ret)
{
    if(root == NULL) return;
    if(ret.size() == depth)
        ret.push_back(vector<int>());
    
    ret[depth].push_back(root->val);
    buildVector(root->left, depth + 1, ret);
    buildVector(root->right, depth + 1, ret);
}

};