【LeetCode】Unique Binary Search Trees
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题目描述:
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解:
只看根节点的左右子树,假设左子树元素数目为i1,右子树为i2,dp表示元素数为i时二叉树的数目,对于i=i1+i2+1,有dp[i]=dp[i1]*dp[i2]。对于给定的n,从第一个元素开始遍历至最后一个元素即可。
代码如下:
class Solution {public: int numTrees(int n) {if (n == 0)return 0;vector<int> count;count.push_back(1);for (int i = 1; i <= n; i++){int num(0);for (int j = 0; j < i / 2; j++)num += count[j] * count[i - j - 1];num *= 2;if (i % 2 == 1)num += count[(i - 1) / 2] * count[(i - 1) / 2];count.push_back(num);}return count[n];}};
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