Different Ways to Add Parentheses leetcode

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]

分治法,类似Unique Binary Search Trees II 每次遇到一个符号就对起左右递归

public class Solution {
    public List<Integer> diffWaysToCompute(String input) {
        List<Integer> res = new ArrayList<Integer>();
        for (int i = 0; i < input.length(); i++) {
            char c = input.charAt(i);
            if (c != '+' && c != '-' && c != '*') {
                continue;
            }
            List<Integer> left = diffWaysToCompute(input.substring(0, i));
            List<Integer> right = diffWaysToCompute(input.substring(i + 1));
            for (int l : left) {
                for (int r : right) {
                    if (c == '+') {
                        res.add(l + r);
                    } else if (c == '-') {
                        res.add(l - r);
                    } else {
                        res.add(l * r);
                    }
                }
            }
            
        }
        if (res.size() == 0) {
            res.add(Integer.parseInt(input));
        }
        return res;
    }
}