剑指 Offer 32 - II. 从上到下打印二叉树 II

题目描述

思路分析

递归迭代均可，递归加上level显示当前深度

代码展示

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

结果分析

时间复杂度O(N)
空间复杂度O(N)