[LeetCode] Basic Calculator Problem Solving report, leetcodecalculator
[Question]
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" 2-1 + 2 " = 3"(1+(4+5+2)-3)+(6+8)" = 23
[Resolution]
Take the test case directly and follow the program below, you will understand its clever points.
public class Solution { public int calculate(String s) { Stack<Integer> stack = new Stack<>(); stack.push(1); stack.push(1); int res = 0; for (int i = 0; i < s.length(); i++) { char c = s.charAt(i); if (Character.isDigit(c)) { int num = c - '0'; int j = i + 1; while (j < s.length() && Character.isDigit(s.charAt(j))) { num = 10 * num + (s.charAt(j) - '0'); j++; } res += stack.pop() * num; i = j - 1; } else if (c == '+' || c == '(') { stack.push(stack.peek()); } else if (c == '-') { stack.push(-1 * stack.peek()); } else if (c == ')') { stack.pop(); } } return res; }}