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[96]不同的二叉搜索树.java

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[96]不同的二叉搜索树.java

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//leetcode submit region begin(Prohibit modification and deletion)

class Solution {

public int numTrees(int n) {

//声明一个备忘录

int[][] memo = new int[n+1][n+1];

return count( memo,1, n);

}

// 定义：闭区间 [lo, hi] 的数字能组成 count(lo, hi) 种 BST

int count(int[][] memo,int lo, int hi) {

if (lo > hi) {

return 1;

}

//先查备忘录

if(memo[lo][hi] != 0){

return memo[lo][hi];

}

int res = 0;

for (int i = lo; i <= hi; i++) {

int left = count(memo,lo, i - 1);

int right = count(memo,i + 1, hi);

res = res + left * right;

memo[lo][hi] = res;

}

return res;

}

}

//leetcode submit region end(Prohibit modification and deletion)