标签：
Implement a basic calculator to evaluate a simple expression string.
The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.
You may assume that the given expression is always valid.
Some examples:
"3+2*2" = 7 " 3/2 " = 1 " 3+5 / 2 " = 5
Note: Do not use the eval built-in library function.
以 3 ＋ 2 ＊ 2为例，这道题的重点是在循环到发现当前字符是＊时，才对2进行处理 （push 2），然后更新sign。
public class Solution { public int calculate(String s) { s = s.replace(" ", ""); int num = 0; char sign = ‘+‘; Stack<Integer> stack = new Stack<>(); for(int i = 0; i < s.length(); ++i) { if(Character.isDigit(s.charAt(i))) { num = num * 10 + (s.charAt(i) - ‘0‘); } if(!Character.isDigit(s.charAt(i)) || i == s.length() - 1) { if(sign == ‘+‘) { stack.push(num); } else if(sign == ‘-‘) { stack.push(-num); } else if(sign == ‘*‘) { stack.push(stack.pop() * num); } else if(sign == ‘/‘) { stack.push(stack.pop() / num);; } sign = s.charAt(i); num = 0; } } int res = 0; for(int ele: stack) { res += ele; } return res; } }
标签：
原文地址：http://www.cnblogs.com/Phoebe815/p/4925026.html
