LeetCode: Unique Binary Search Trees

有经验了，一上来就知道用dp， 一次过

 1 class Solution {

 2 public:

 3     int numTrees(int n) {

 4         // Start typing your C/C++ solution below

 5         // DO NOT write int main() function

 6         if (n < 2) return 1;

 7         vector<int> f(n+1, 0);

 8         f[0] = 1;

 9         f[1] = 1;

10         for (int i = 2; i <= n; i++) {

11             for (int j = 0; j < i; j++)

12                 f[i] += f[j]*f[i-1-j];

13         }

14         return f[n];

15     }

16 };

 C#

 1 public class Solution {

 2     public int NumTrees(int n) {

 3         if (n < 2) return 1;

 4         int[] f = new int[n+1];

 5         f[0] = f[1] = 1;

 6         for (int i = 2; i <= n; i++) {

 7             for (int j = 0; j < i; j++) {

 8                 f[i] += f[j] * f[i-1-j];

 9             }

10         }

11         return f[n];

12     }

13 }

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