Basic Calculator II.java

// Runtime: 33 ms (Top 27.28%) | Memory: 45.6 MB (Top 25.17%)
class Solution {
    public int calculate(String s) {
        if(s==null ||s.length()==0)return 0;
        Stack<Integer> st = new Stack<>();
        int curr=0;
        char op = '+';
        char [] ch = s.toCharArray();
        for(int i=0;i<s.length();i++){
            if(Character.isDigit(ch[i])){
                curr= curr*10+ch[i]-'0';
            }
            if(!Character.isDigit(ch[i])&& ch[i]!=' ' || i==ch.length-1){
                if(op=='+'){
                    st.push(curr);
                }else if(op=='-'){
                    st.push(-curr);
                }else if(op=='*'){
                    st.push(st.pop()*curr);
                }else if(op=='/'){
                    st.push(st.pop()/curr);
                }
                op=ch[i];
                curr=0;
            }
        }
        int sum=0;
        while(!st.isEmpty()){
            sum+=st.pop();
        }
        return sum;
    }
} //TC=o(n),SC=o(n)