LC 416. Partition Equal Subset Sum

题目

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:

1. Each of the array element will not exceed 100.
2. The array size will not exceed 200.

Example 1:

Input: [1, 5, 11, 5]
Output: true
Explanation: The array can be partitioned as [1, 5, 5] and [11].

Example 2:

Input: [1, 2, 3, 5]
Output: false
Explanation: The array cannot be partitioned into equal sum subsets.

参考答案

class Solution {
public:
    bool canPartition(vector<int>& nums) {
        int sum = accumulate(nums.begin(),nums.end(),0); //首先对数组求和
        if( sum & 1 ){ // 如果数组是个奇数，那么不成立，因为例如: 5 = 2+3
            return false;
        }
        int half = sum / 2;
        std::sort(nums.begin(),nums.end(),std::greater<int>()); // 一定要加，否则time limit exceeded
        return foo(nums,half,0); // 将 half 作为输入
    }

    bool foo(vector<int>& nums, int half, int index){
        for(int i = index ; i <nums.size() ; i++){ // 对于每一个数进行迭代
            int h = half - nums[i]; // 对 half 扣除
            if(h<0) return false; 
            if(h==0) return true;
            // 如果上述条件都不满足，说明nums[i] 没办法满足条件，那么需要继续找下一个数，
20　　　 　　　　// 即index = i+1。满足即true，不满足就退回到上层，在这一层找下一个数。
            if(foo(nums,h,i+1) == true) return true; 
        }
        return false;
    }
};