96. 不同的二叉搜索树

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class Solution {
public:
    int numTrees(int n) {
        std::vector visited(n+1, -1);
        return dfs(n, visited);
    }
private:
    int dfs(int n, std::vector& visited) {
        if (n == 0 || n == 1) {
            return 1;
        }
        if (visited[n] != -1) {
            return visited[n];
        }
        int result = 0;
        for (int i = 1; i <= n; ++i) {
            int tmp = dfs(i-1, visited) * dfs(n-i, visited);
            result += tmp;
        }
        visited[n] = result;
        return result;
    }
};