[Leetcode] Unique Binary Search Trees

class Solution {
public:
    int numTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (n == 0) return 1;
        int sum = 0;
        for (int i = 0; i < n; ++i)
        {
            sum += numTrees(i) * numTrees(n - i - 1);
        }
        
        return sum;
    }
};

class Solution {
public:
    int numTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (n <= 1) return n;
        
        vector<int> ways(n + 1);
        
        ways[0] = 1;
        ways[1] = 1;
        for (int i = 2; i <= n; ++i)
        {
            ways[i] = 0;
            
            for (int j = 0; j < i; ++j)
            {
                int l = ways[j];
                int r = ways[i - j - 1];
                ways[i] += l * r;
            }
        }

        return ways[n];
    }
};