[LeetCode] Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

 

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　　实现方法有两种，一种是二叉树自下往上 实现，一种是自上往下实现。我用的是后一种方法。

 

#include <iostream>

#include <vector>

using namespace std;

/**

 * Definition for binary tree

 */

struct TreeNode {

    int val;

    TreeNode *left;

    TreeNode *right;

    TreeNode(int x) : val(x), left(NULL), right(NULL) {}

};



class Solution {

public:

    TreeNode *sortedArrayToBST(vector<int> &num) {

        int n = num.size();

        if(n ==0)   return NULL;

        if(n==1)    return new TreeNode(num[0]);

        return helpFun(num,0,n-1);

    }



    TreeNode * helpFun(vector<int> &num,int lft,int rgt)

    {

        if(lft>rgt) return NULL;

        if(lft==rgt)    return new TreeNode(num[lft]);

        int mid = (lft+rgt)/2;

        TreeNode * curNode = new TreeNode(num[mid]);

        curNode->left = helpFun(num,lft,mid-1);

        curNode->right = helpFun(num,mid+1,rgt);

        return curNode;

    }

};



int main()

{



    return 0;

}