Description:
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:
- 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.
这道题要用到计算所有有可能出现的情况然后再比较,用递归可以做。时间复杂度为O(n^2)。
class Solution {
public:
int result ;
int findTargetSumWays(vector<int>& nums, int S) {
dfs(0,0,nums,S);
return result;
}
void dfs(int sum, int count, vector<int>&nums, int s){
if(count == nums.size()){
if(sum == s) result++;
return;
}
dfs(sum+nums[count], count+1, nums, s);
dfs(sum-nums[count], count+1, nums, s);
}
};