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.
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
Solutions
Solution 1: Dynamic Programming
Let's denote the sum of all elements in the array $\textit{nums}$ as $s$, and the sum of elements to which we assign a negative sign as $x$. Therefore, the sum of elements with a positive sign is $s - x$. We have:
$$
(s - x) - x = \textit{target} \Rightarrow x = \frac{s - \textit{target}}{2}
$$
Since $x \geq 0$ and $x$ must be an integer, it follows that $s \geq \textit{target}$ and $s - \textit{target}$ must be even. If these two conditions are not met, we directly return $0$.
Next, we can transform the problem into: selecting several elements from the array $\textit{nums}$ such that the sum of these elements equals $\frac{s - \textit{target}}{2}$. We are asked how many ways there are to make such a selection.
We can use dynamic programming to solve this problem. Define $f[i][j]$ as the number of ways to select several elements from the first $i$ elements of the array $\textit{nums}$ such that the sum of these elements equals $j$.
For $\textit{nums}[i - 1]$, we have two choices: to select or not to select. If we do not select $\textit{nums}[i - 1]$, then $f[i][j] = f[i - 1][j]$; if we do select $\textit{nums}[i - 1]$, then $f[i][j] = f[i - 1][j - \textit{nums}[i - 1]]$. Therefore, the state transition equation is:
We can observe that in the state transition equation of Solution 1, the value of $f[i][j]$ is only related to $f[i - 1][j]$ and $f[i - 1][j - \textit{nums}[i - 1]]$. Therefore, we can eliminate the first dimension of the space and use only a one-dimensional array.
The time complexity is $O(m \times n)$, and the space complexity is $O(n)$.