494. Target Sum Medium

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.

思路：本题使用动态规划以及深度优先搜索的思想，对数组中的每一个元素分为正号和负号，递归进行相加，判断最终结果与S是否相同。

void help(vector<int>&nums, int S, int &count, int index, int now_sum) {    if (index >= nums.size()) {        if (now_sum == S) {            count++;        }        return;    }    now_sum += nums[index];    index++;    help(nums, S, count, index, now_sum);    index--;    now_sum -= nums[index];    now_sum -= nums[index];    index++;    help(nums, S, count, index, now_sum);}class Solution {public:    int findTargetSumWays(vector<int>& nums, int S) {        if (nums.size() == 0) {            return 0;        }        int count = 0, index = 0, now_sum = 0;        help(nums, S, count, index, now_sum);        return count;    }};