494. Target Sum

You are given an integer array nums and an integer target.

You want to build an expression out of nums by adding one of the symbols ‘+’ and ‘-’ before each integer in nums and then concatenate all the integers.

For example, if nums = [2, 1], you can add a ‘+’ before 2 and a ‘-’ before 1 and concatenate them to build the expression “+2-1”.

Return the number of different expressions that you can build, which evaluates to target.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
Example 1:

Input: nums = [1,1,1,1,1], target = 3
Output: 5
Explanation: There are 5 ways to assign symbols to make the sum of nums be target 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
+1 + 1 + 1 + 1 - 1 = 3

Example 2:

Input: nums = [1], target = 1
Output: 1

Constraints:

1 <= nums.length <= 20
0 <= nums[i] <= 1000
0 <= sum(nums[i]) <= 1000
-1000 <= target <= 1000

Solution

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
class Solution {
    int count = 0;
    public int findTargetSumWays(int[] nums, int target) {
        search(nums, 0, 0, target);
        return count;
    }
    
    void search(int[] nums, int start, int sum, int target) {
        if (target == sum && start == nums.length) {
            count++;
            return;
        }
        
        if (start >= nums.length) return;
        
        search(nums, start + 1, sum + nums[start], target);
        search(nums, start + 1, sum - nums[start], target);
    }
}

Solution#

Solution