Target Sum

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Problem

Given an array of non-negative integers and a target, assign either a + or a - sign to each number and count how many ways the resulting expression evaluates to the target.

Examples

Example 1

Input:nums = [1,1,1,1,1], target = 3

Output:5

Constraints

1 <= nums.length <= 20

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How to approach it: the 2-D Dynamic Programming pattern

The same DP idea as 1-D, but the state depends on two indices — often two positions in a grid, or a position in each of two sequences being compared.

Look for this pattern when

You're comparing two sequences (finding a longest common subsequence, edit distance).
You're computing paths or optimal values across a 2-D grid.

Read the full 2-D Dynamic Programming guide →

Video walkthroughs

Original problem on LeetCode ↗