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.
Example 1:
Input: "3+2*2" Output: 7
Example 2:
Input: " 3/2 " Output: 1
Example 3:
Input: " 3+5 / 2 " Output: 5
Note:
- You may assume that the given expression is always valid.
- Do not use the
eval
built-in library function.
思路:这道题厉害的样例是“100000/8/7/6”,导致算式必须从左向右算。另外,要切分数字。
所以,第一次遍历时只求出每一段的和,第二次则把部分和累加。
class Solution { public int calculate(String s) { Stack<Integer> stack = new Stack<Integer>(); char symbol = '+'; char[] nums = s.toCharArray(); StringBuilder sb = new StringBuilder(); for (int i = 0; i < nums.length; i++) { if (nums[i] <= '9' && nums[i] >= '0') { sb.append(nums[i]); if (i == nums.length - 1) { int num2 = Integer.valueOf(sb.toString()); if (symbol == '+') { stack.push(num2); } else if (symbol == '-') { stack.push(-num2); } else if (symbol == '*') { int num1 = stack.pop(); stack.push(num1 * num2); } else { int num1 = stack.pop(); stack.push(num1 / num2); } } } else if (nums[i] == ' ') { if (i == nums.length - 1) { int num2 = Integer.valueOf(sb.toString()); if (symbol == '+') { stack.push(num2); } else if (symbol == '-') { stack.push(-num2); } else if (symbol == '*') { int num1 = stack.pop(); stack.push(num1 * num2); } else { int num1 = stack.pop(); stack.push(num1 / num2); } } continue; } else { int num2 = Integer.valueOf(sb.toString()); sb = new StringBuilder(); if (symbol == '+') { stack.push(num2); } else if (symbol == '-') { stack.push(-num2); } else if (symbol == '*') { int num1 = stack.pop(); stack.push(num1 * num2); } else { int num1 = stack.pop(); stack.push(num1 / num2); } symbol = nums[i]; } } int ans = 0; for(int num : stack){ ans += num; } return ans; } }