leetcode/stack/Basic_Calculator.java at master · Aayers-ghw/leetcode · GitHub

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package stack;

import java.util.Stack;

public class Basic_Calculator {

	public int calculate(String s) {
        Stack<Integer> st = new Stack<Integer>();
        int res = 0, sign = 1;
        for(int i = 0; i < s.length(); ++i){
            if(Character.isDigit(s.charAt(i))){
                int sum = s.charAt(i) - '0';
                while(i + 1 < s.length() && Character.isDigit(s.charAt(i + 1))){
                    sum = sum * 10 + s.charAt(i + 1) - '0';
                    ++i;
                }
                res += sign * sum;
            }else if(s.charAt(i) == '+'){
                sign = 1;
            }else if(s.charAt(i) == '-'){
                sign = -1;
            }else if(s.charAt(i) == '('){
                st.push(res);
                st.push(sign);
                res = 0;
                sign = 1;
            }else if(s.charAt(i) == ')'){
                res *= st.pop();
                res += st.pop();
            }
        }
        return res;
    }
}