【转载】判断整数序列是不是二元查找树的后序遍历结果

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。
ANSWER:
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 = i;  while (i>=s && a[e]<a[i]) i--;    if (i < s)    return helper(a, s, k);  else    return 0;}

Maybe another solution is also fine:

int helper(int a[], int s, int e, int max, int min) Similar to http://blog.csdn.net/taoqick/article/details/16992475