#494 - Target Sum

Medium • View on LeetCode • View on GitHub

Problem Description

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.

Solution

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number}
 */
var findTargetSumWays = function(nums, target) {
  const map = new Map();

  function dp(index, sum) {
    if (index === nums.length) {
      return sum === target ? 1 : 0;
    }
    const key = `${index},${sum}`;
    if (map.has(key)) {
      return map.get(key);
    }
    const r = dp(index + 1, sum + nums[index]) + dp(index + 1, sum - nums[index]);
    map.set(key, r);
    return r;
  }

  return dp(0, 0);
};