Maximum Binary Tree

遞歸求解，每次找出最的的元素作爲樹根，思路有點類似快排

class Solution {
    
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
public:
      TreeNode* sub_construct(vector<int>& nums , int start,int end){
        if(start>end) return NULL;
        int max = -1;
        int flag;
        for(int i=start;i<=end;i++){
            if(nums[i]>max){
                max = nums[i];
                flag = i;
            }
        }
        TreeNode* root = new TreeNode(nums[flag]);
        root->left = sub_construct(nums,start,flag-1);
        root->right = sub_construct(nums,flag+1,end);
        return root;
    }
    
    
    TreeNode* constructMaximumBinaryTree(vector<int>& nums) {
        if(nums.size() == 0) return NULL;
        else return sub_construct(nums,0,nums.size()-1);    
    }
};