leetcode-Basic Calculator II

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.

解析：

代码解析：

package leetcode;
import java.util.Stack;

//不存符号，只存每一步的结果；
//主要思路：将+-当作数字的正负，将其push到栈中（最后出栈的时候一起按加法处理，优先级靠后，所以后运算）；
//当遇到*/时，仅把当前num和stack中的最后进入的num相乘，结果继续放入stack中（/也一样）（优先级靠前，所以先运算）。
//符合这个场景  --> stack
public class BasicCalculator {
    public static 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);
                }

//注意pop和push的方法
                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;
    }
    public static void main(String[] args) {
        System.out.print(calculate("3+4-5*6/7"));
    }
}