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
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class Solution {
public int findTargetSumWays(int[] nums, int target) {
return waysHelper(nums, target, 0, new HashMap<String, Integer>(), 0);
}
public int waysHelper(int[] nums, int target, int idx, Map<String, Integer> map, int total){
if(idx == nums.length){
if(total == target) return 1;
return 0;
}
if(!map.containsKey(idx+":"+total)){
int x = waysHelper(nums, target, idx+1, map, total + nums[idx]);
int y = waysHelper(nums, target, idx+1, map, total - nums[idx]);
int t = x + y;
map.put(idx+":"+total, t);
}
return map.get(idx+":"+total);
}
}