题目描述
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:
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:
- The length of the given array is positive and will not exceed 20.
- The sum of elements in the given array will not exceed 1000.
- Your output answer is guaranteed to be fitted in a 32-bit integer.
解题思路
- 深度优先搜索
- 首先判断一个指针，是否走到了nums的末尾， 注意，由于是末尾，指针应该是等于size
- 然后判断是否与指定的值相等
- 当前的值，加上或者减去nums[begin]即可
C++代码
class Solution {
public:
int findTargetSumWays(vector<int>& nums, int S) {
int res = 0;
dfs(nums, res, 0, S, 0);
return res;
}
void dfs(vector<int>& nums, int& res, int begin, int target, int cur){
if(begin == nums.size()){
if(cur == target){
res++;
}
return;
}
dfs(nums, res, begin + 1, target, cur + nums[begin]);
dfs(nums, res, begin + 1, target, cur - nums[begin]);
}
};
