102. 二叉树的层次遍历

给定一个二叉树，返回其按层次遍历的节点值。 （即逐层地，从左到右访问所有节点）。例如:

给定二叉树: [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回其层次遍历结果：

[
  [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) {
        int depth = getHight(root);
        vector<vector<int>> result(depth);
        if(depth <= 0)
            return result;
        levelOfOrder(root, 0, result);
        return result;
    }
    void levelOfOrder(TreeNode* root, int depth, vector<vector<int>> &result)
    {
        if(root == NULL)
            return ;
        result[depth].push_back(root->val);
        levelOfOrder(root->left, depth + 1, result);
        levelOfOrder(root->right, depth + 1, result);
    }
    int getHight(TreeNode* root)
    {
        if(root == NULL)
            return 0;
        int L = getHight(root->left);
        int R = getHight(root->right);
        return max(L, R) + 1;
    }
};