LeetCode – Basic Calculator (Java)

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces.You may assume that the given expression is always valid.

Some examples: "1 + 1" = 2, "(1)" = 1, "(1-(4-5))" = 2

Analysis

This problem can be solved by using a stack. We keep pushing element to the stack, when ')" is met, calculate the expression up to the first "(".

Java Solution

public int calculate(String s) {
	// delte white spaces
	s = s.replaceAll(" ", "");
 
	Stack<String> stack = new Stack<String>();
	char[] arr = s.toCharArray();
 
	StringBuilder sb = new StringBuilder();
	for (int i = 0; i < arr.length; i++) {
		if (arr[i] == ' ')
			continue;
 
		if (arr[i] >= '0' && arr[i] <= '9') {
			sb.append(arr[i]);
 
			if (i == arr.length - 1) {
				stack.push(sb.toString());
			}
		} else {
			if (sb.length() > 0) {
				stack.push(sb.toString());
				sb = new StringBuilder();
			}
 
			if (arr[i] != ')') {
				stack.push(new String(new char[] { arr[i] }));
			} else {
				// when meet ')', pop and calculate
				ArrayList<String> t = new ArrayList<String>();
				while (!stack.isEmpty()) {
					String top = stack.pop();
					if (top.equals("(")) {
						break;
					} else {
						t.add(0, top);
					}
				}
 
				int temp = 0;
				if (t.size() == 1) {
					temp = Integer.valueOf(t.get(0));
				} else {
					for (int j = t.size() - 1; j > 0; j = j - 2) {
						if (t.get(j - 1).equals("-")) {
							temp += 0 - Integer.valueOf(t.get(j));
						} else {
							temp += Integer.valueOf(t.get(j));
						}
					}
					temp += Integer.valueOf(t.get(0));
				}
				stack.push(String.valueOf(temp));
			}
		}
	}
 
	ArrayList<String> t = new ArrayList<String>();
	while (!stack.isEmpty()) {
		String elem = stack.pop();
		t.add(0, elem);
	}
 
	int temp = 0;
	for (int i = t.size() - 1; i > 0; i = i - 2) {
		if (t.get(i - 1).equals("-")) {
			temp += 0 - Integer.valueOf(t.get(i));
		} else {
			temp += Integer.valueOf(t.get(i));
		}
	}
	temp += Integer.valueOf(t.get(0));
 
	return temp;
}
Category >> Algorithms >> Interview  
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  • Umesh Singla

    That will assume only digits in the string, but we can have any integer in the string like “1+12”.

  • Liu Xuemei

    why not converting to reverse polish notation first, then calculate the value