224.Basic-Calculator

224. Basic Calculator

题目地址

https://leetcode.com/problems/basic-calculator/description/

题目描述

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

Example 1:
Input: "1 + 1"
Output: 2

Example 2:
Input: " 2-1 + 2 "
Output: 3

Example 3:
Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23

Note:
You may assume that the given expression is always valid.
Do not use the eval built-in library function.

代码

Approach #1

class Solution {
    public int calculate(String s) {
    int len = s.length(), sign = 1, result = 0;
    Stack<Integer> stack = new Stack<Integer>();
    for (int i = 0; i < len; i++) {
      if (Character.isDigit(s.charAt(i))) {
        int num = s.charAt(i) - '0';
        while (i + 1 < len && Character.isDigit(s.charAt(i + 1))) {
          num = num * 10 + s.charAt(i + 1) - '0';
          i++;
        }
        result += num * sign;
      } else if (s.charAt(i) == '+') {
        sign = 1;
      } else if (s.charAt(i) == '-') {
        sign = -1;
      } else if (s.charAt(i) == '(') {
        stack.push(result);
        stack.push(sign);
        result = 0;
        sign = 1;
      } else if (s.charAt(i) == ')') {
        result = result * stack.pop() + stack.pop();
      }
    }

    return result;
    }
}