Leetcode 241 Different Ways to Add Parentheses

Given a string of numbers and operators, return all possible results from computing all the different possible ways to group numbers and operators. The valid operators are+, - and *.

Example 1

Input: "2-1-1".

((2-1)-1) = 0
(2-(1-1)) = 2

Output: [0, 2]

Example 2

Input: "2*3-4*5"

(2*(3-(4*5))) = -34
((2*3)-(4*5)) = -14
((2*(3-4))*5) = -10
(2*((3-4)*5)) = -10
(((2*3)-4)*5) = 10

Output: [-34, -14, -10, -10, 10]

Solution 1: Since parentheses can be added anywhere, use divide and conquer.

 public class Solution {  
   public List<Integer> diffWaysToCompute(String input) {  
     int n=input.length();  
     List<Integer> res=new ArrayList<>();  
     for (int i=0; i<n; i++) {  
       char c=input.charAt(i);  
       if (c=='+' || c=='-' || c=='*') {  
         List<Integer> left=diffWaysToCompute(input.substring(0,i));  
         List<Integer> right=diffWaysToCompute(input.substring(i+1));  
         for (int n1: left) {  
           for (int n2: right) {  
             if (c=='+') res.add(n1+n2);  
             else if (c=='-') res.add(n1-n2);  
             else if (c=='*') res.add(n1*n2);  
           }  
         }  
       }  
     }  
     if (res.size()==0) res.add(Integer.valueOf(input));  
     return res;  
   }  
 }