*LeetCode-Unique Binary Search Trees
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卡特兰数的应用之一,还有一个是让求有多少种方式配对括号。
http://en.wikipedia.org/wiki/Catalan_number
这个题发现对于每个数字n 解法数=左子树0个结点的方法 * 右子树n-1个节点的方法 + 左1个节点*右子n-2个.......
要注意的是要多存一个arr[0] = 1 数组是n+1大小的
public class Solution { public int numTrees(int n) { if ( n <= 0 ) return 0; int [] arr = new int [n+1]; arr[0] = 1; for ( int i = 1; i < n+1; i++ ){ for ( int j = 0; j < i; j ++){ arr[i] += arr[j]*arr[i-j-1]; } } return arr[n]; }}
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