Given a non-empty array containing only positive integers, find if the array can be partitioned into two subsets such that the sum of elements in both subsets is equal.

Note:
Each of the array element will not exceed 100.
The array size will not exceed 200.
这道题目，考察coder思维的缜密性，不能漏过任何一种情况。
思路：
1.预处理，将数组排列称为降序排列。
2.初判断，sum是否为奇数、首元素是否大于sum/2。
3.递归，二分向下，对于每个元素都分选取，和不选取两种情况。

    bool canPartition(vector<int>& nums) {
        int sum = 0;
        for(int i =0;i<nums.size();i++){
            sum+= nums[i];
        }
        if(sum%2) return false;
        sum /= 2;
        sort(nums.rbegin(),nums.rend());
        return helper(nums, sum, 0);
    }
    bool helper(vector<int>& nums, int sum, int index){
        if(sum == nums[index]) return true;
        if(sum < nums[index]) return false;
        return helper(nums,sum-nums[index],index+1) || helper(nums,sum,index+1);
    }