9. 判断整数序列是不是二元查找树的后序遍历结果

题目：输入一个整数数组，判断该数组是不是某二元查找树的后序遍历的结果。
如果是返回true，否则返回false。
例如输入5、7、6、9、11、10、8，由于这一整数序列是如下树的后序遍历结果：
     8
    / \
  6   10
 / \    / \
5 7 9 11
因此返回true。
如果输入7、4、6、5，没有哪棵树的后序遍历的结果是这个序列，因此返回false。

HANDWRITING:

不会。。

ANSWER FROM:http://blog.csdn.net/v_july_v/article/details/6870251
This is an interesting one. There is a traditional question that requires the binary tree to be re-constructed from mid/post/pre order results. This seems similar. For the
problems related to (binary) trees, recursion is the first choice. 
In this problem, we know in post-order results, the last number should be the root. So we have known the root of the BST is 8 in the example. So we can split the array by
the root. 

int isPostorderResult(int a[], int n) {
return helper(a, 0, n-1);
}
int helper(int a[], int s, int e) {
if (e==s) return 1;
int i=e-1;
while (a[e]>a[i] && i>=s) i--;
if (!helper(a, i+1, e-1))
return 0;
int k = l;
while (a[e]<a[i] && i>=s) i--;
return helper(a, s, l);
}

FROM:http://blog.csdn.net/v_july_v/article/details/6870251