leetcode654 Maximum Binary Tree

思路：

使用单调栈可以达到O(n)。

实现：

 /**
  * Definition for a binary tree node.
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution
 {
 public:
     TreeNode* constructMaximumBinaryTree(vector<int>& nums)
     {
         stack<TreeNode*> st;
         for (auto it: nums)
         {
             TreeNode* tmp = new TreeNode(it), *last = NULL;
             while (!st.empty() && tmp->val > st.top()->val)
             {
                 last = st.top(); st.pop();
             }
             tmp->left = last;
             if (!st.empty()) st.top()->right = tmp;
             st.push(tmp);
         }
         TreeNode* res = NULL;
         while (!st.empty())
         {
             res = st.top(); st.pop();
         }
         return res;
     }
 }