Leetcode: Unique Binary Search Trees

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

找个简单的，睡前保持好心情。

class Solution {
public:
    int numTrees(int n) {
        if (n < 0) {
            return 0;
        }
        if (n == 0 || n == 1) {
            return 1;
        }
        
        int sum = 0;
        // Each value can be the root
        for (int i = 1; i <= n; i++) {
            sum += numTrees(i-1) * numTrees(n-i);
        }
        
        return sum;
    }
};

=========================第二次====================

class Solution {
public:
int numTrees(int n) {
if (n < 0) {
return 0;
}
else if (n == 0 || n == 1) {
return 1;
}
else {
int result = 0;
for (int i = 0; i < n; ++i) {
result += numTrees(i) * numTrees(n-i-1);
}
return result;
}
}
};