494. Target Sum

You are given a list of non-negative integers, a1, a2, ..., an, and a target, S. Now you have 2 symbols + and -. For each integer, you should choose one from + and - as its new symbol.

Find out how many ways to assign symbols to make sum of integers equal to target S.

Example 1:

Input: nums is [1, 1, 1, 1, 1], S is 3. 
Output: 5
Explanation: 

-1+1+1+1+1 = 3
+1-1+1+1+1 = 3
+1+1-1+1+1 = 3
+1+1+1-1+1 = 3
+1+1+1+1-1 = 3

There are 5 ways to assign symbols to make the sum of nums be target 3.

Note:

1. The length of the given array is positive and will not exceed 20.
2. The sum of elements in the given array will not exceed 1000.
3. Your output answer is guaranteed to be fitted in a 32-bit integer.

 方法1：递归暴力解决（效果较差，仅比6%的人快）

class Solution {
public:

    void help(vector<int>& nums, int i, int sum, int target, int &count)
    {
        if (i + 1 == nums.size() && sum == target)count++;
        else if (i + 1 == nums.size() && sum != target)return ;
        else
        {
            help(nums, i + 1, sum + nums[i + 1], target, count);
            help(nums, i + 1, sum - nums[i + 1], target, count);
        }
    }
    int findTargetSumWays(vector<int>& nums, int S) {
        if (nums.size() < 1) return 0;
        int count = 0;
        help(nums, 0, nums[0], S, count);
        help(nums, 0, -nums[0], S, count);
        return count;
    }
};

方法2：动态规划（没看懂，待学习）