LeetCode – Basic Calculator II (Java)

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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

Java Solution

public int calculate(String s) {
    int md=-1; // 0 is m, 1 is d
    int sign=1; // 1 is +, -1 is -
    int prev=0;
    int result=0;
 
    for(int i=0; i<s.length(); i++){
        char c = s.charAt(i);
        if(Character.isDigit(c)){
            int num = c-'0';
            while(++i<s.length() && Character.isDigit(s.charAt(i))){
                num = num*10+s.charAt(i)-'0';
            }
            i--; // back to last digit of number
 
            if(md==0){
                prev = prev * num;
                md=-1;
            }else if(md==1){
                prev = prev / num;
                md=-1;
            }else{
                prev = num;
            }
        }else if(c=='/'){
            md=1;
        }else if(c=='*'){
            md=0;
        }else if(c=='+'){
            result = result + sign*prev;
            sign=1;
        }else if(c=='-'){
            result = result + sign*prev;
            sign=-1;
        }
    }
 
    result = result + sign*prev;
    return result;
}

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