leetcode 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.

思路：

类似背包问题。

class Solution {public:    bool canPartition(vector<int>& nums) {        int sum = 0;        for(auto val:nums) sum+=val;        if(sum%2==1) return false;        vector<int> dp(sum+1,0);        dp[0] = 1;        for(int i = 0; i < nums.size(); i++) {            for(int j = sum; j >=nums[i] ; j --){                if(dp[j-nums[i]]!=0) dp[j] =1;                else dp[j] = dp[j];            }        }        return dp[sum/2] != 0;    }};

别人的：

class Solution {public:    bool canPartition(vector<int>& nums) {        bitset<10001> bits(1);        int sum = accumulate(nums.begin(), nums.end(), 0);        for (auto n : nums) bits |= bits << n;        return !(sum & 1) && bits[sum >> 1];    }};

时间最优：

bool myfunction(int a, int b) {return a>b;}class Solution {public:    bool canPartition(vector<int>& nums) {        int sum = 0;        for (int num : nums) sum+=num;    //    cout << "size is " << nums.size() << "; sum is " << sum << endl;        if (sum %2 !=0) return false;        sort(nums.begin(),nums.end(),myfunction);        if (nums[0]>sum/2) return false;       return  canPartitionRecursive(nums, sum/2, 0);    }        bool canPartitionRecursive(vector<int>& nums, int target, int curIdx)    {     //   cout << "target is " << target << "; current index is " << curIdx << endl;        if (target<=0) return target==0;        for (int i = curIdx; i < nums.size(); i++)        {            if (nums[i]>target) continue;            if (canPartitionRecursive(nums, target-nums[i],i+1))                return true;        }        return false;    }

};