Different Ways to Add Parentheses
題意：
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]
解題思路：
使用遞迴去作，一遇到operator就把字串切成兩半分別遞迴，之後再根字operator把所有回傳的結果組合起來，再往上一層傳。
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 == '*') {
String strA = input.substring(0, i);
String strB = input.substring(i + 1);
List<Integer> partA = diffWaysToCompute(strA);
List<Integer> partB = diffWaysToCompute(strB);
for (int a : partA) {
for (int b : partB) {
if (c == '+') {
res.add(a + b);
} else if (c == '-') {
res.add(a - b);
} else if (c == '*') {
res.add(a * b);
}
}
}
}
}
if (res.size() == 0) res.add(Integer.valueOf(input));
return res;
}
}