June Leetcoding Challenge(6.24)

LeetCode六月挑战（6.24 ）Unique Binary Search Trees LeetCode 96解题方案

题目描述

Given n, how many structurally unique BST’s (binary search trees) that store values 1 … n?

Example:

Input: 3
Output: 5
Explanation:
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

思路

代码

class Solution {
    public int numTrees(int n) {
        if(n<1)
        return 0;
        int[][] dp=new int[n+1][n+1];
        return f(1,n,dp);
    }
    public int f(int i,int j,int[][] dp){
        if(i==j || i>j) return 1;
        if(dp[i][j]!=0) return dp[i][j];
        int sum=0;
        for(int x=i;x<=j;x++){
            sum+=f(i,x-1,dp)*f(x+1,j,dp);
        }
        dp[i][j]=sum;
        return dp[i][j];
    }
}