96-Unique Binary Search Trees
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题目
Given n, how many structurally unique BST’s (binary search trees) that store values 1…n?
For example,
Given n = 3, there are a total of 5 unique BST’s.
1 3 3 2 1 \ / / / \ \ 3 2 1 1 3 2 / / \ \ 2 1 2 3
分析
- 二叉搜索树的性质:左边数字比根节点小,右边数字比根节点大
- 二叉树的数量是左子树的数量乘以又子树的数量,而且左右子树的结构数量与数字无关,跟节点数目有关
- 根节点可以选择1-n的任意数字,二叉树种类数目是以根节点为1-n的二叉树种类相加
实现
class Solution {public: int numTrees(int n) { vector<int> dp(n + 1, 0); dp[0] = dp[1] = 1; for (int i = 2; i < n + 1; i++) for (int j = 0; j < i; j++) dp[i] += dp[j] * dp[i - j - 1]; return dp[n]; }};
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