227. Basic Calculator II**

最新推荐文章于 2024-05-01 11:04:39 发布

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最新推荐文章于 2024-05-01 11:04:39 发布

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分类专栏： leetcode Java String

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本文介绍了一个简易计算器的设计与实现，该计算器能处理简单的算术表达式，包括加、减、乘、除运算，并通过堆栈数据结构来实现有效的整数运算。

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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.

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

    int re = 0;
    for(int i:stack){
        re += i;
    }
    return re;
}
}