该题要根据中缀表达式算出结果。这题参考了链接：
https://blog.csdn.net/tc_to_top/article/details/52348800
的方法2编写的，主要思路是递归。
代码如下：
1 class Solution {
2 public int calculate(String s) {
3 if(s.length() == 0){
4 return 0;
5 }
6
7 s = "(" + s + ")";
8 int[] pos = {0};
9
10 return eval(s, pos);
11 }
12
13 private static int eval(String s, int[] pos){
14 int val = 0, i = pos[0], sign = 1, num = 0;
15
16 while(i < s.length()){
17 char c=s.charAt(i);
18 switch(c){
19 case '+':val = val + sign*num; num = 0; sign = 1; i++; break;
20 case '-':val = val + sign*num; num = 0; sign =-1; i++; break;
21 case '(':pos[0] = i + 1; val = val + sign*eval(s, pos); i = pos[0]; break;
22 case ')':pos[0] = i + 1; return val + sign*num;
23 case ' ':i++; continue;
24 default:num = num * 10 + c - '0'; i++;
25 }
26 }
27 return val;
28 }
29 }
END