Leetcode习题：Unique Binary Search Trees

Unique Binary Search Trees

AC Rate: 1802/5241
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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

有些小细节得多注意，比如设置初始值，没有判断n的大小，就给n=2进行赋值。在n=1时就错了……但想想其实n=2时也不需要赋值。

public class Solution {
public int numTrees(int n) {
int[] num = new int[n+1];

num[0] = 1;
num[1] = 1;

for(int i = 2; i<=n; i++){
int sum = 0;
for(int j = 1; j<=i; j++){
sum += num[j-1]*num[i-j];
}
num[i] = sum;
}

return num
;
}
}