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);
}
};
