题解 | 表达式求值

import java.util.*;

public class Solution {
    /**
     * 返回表达式的值
     * @param s string字符串 待计算的表达式
     * @return int整型
     */
    public int solve (String s) {
        Stack<Integer> nums = new Stack<>();  // 数字栈
        Stack<Character> ops = new Stack<>(); // 运算符栈

        int n = s.length();
        int i = 0;

        while (i < n) {
            char c = s.charAt(i);

            if (c == ' ') { // 跳过空格
                i++;
                continue;
            }

            if (Character.isDigit(c)) {
                // 读完整的数字
                int num = 0;
                while (i < n && Character.isDigit(s.charAt(i))) {
                    num = num * 10 + (s.charAt(i) - '0');
                    i++;
                }
                nums.push(num);
                continue; // 注意这里不能 i++，因为已经在 while 里前进
            }

            if (c == '(') {
                ops.push(c);
                i++;
            } else if (c == ')') {
                // 计算直到遇到左括号
                while (!ops.isEmpty() && ops.peek() != '(') {
                    calc(nums, ops);
                }
                ops.pop(); // 弹出 '('
                i++;
            } else { // 运算符 + - *
                while (!ops.isEmpty() && ops.peek() != '(' 
                       && priority(ops.peek()) >= priority(c)) {
                    calc(nums, ops);
                }
                ops.push(c);
                i++;
            }
        }

        // 处理剩余运算符
        while (!ops.isEmpty()) {
            calc(nums, ops);
        }

        return nums.pop();
    }

    // 计算一次
    private void calc(Stack<Integer> nums, Stack<Character> ops) {
        int b = nums.pop();
        int a = nums.pop();
        char op = ops.pop();
        int res = 0;
        if (op == '+') res = a + b;
        else if (op == '-') res = a - b;
        else if (op == '*') res = a * b;
        nums.push(res);
    }

    // 运算符优先级
    private int priority(char op) {
        if (op == '+' || op == '-') return 1;
        if (op == '*') return 2;
        return 0;
    }

    public static void main(String[] args) {
        Solution s = new Solution();
        System.out.println(s.solve("2+3*4"));      // 14
        System.out.println(s.solve("(2+3)*4"));    // 20
        System.out.println(s.solve("100-50*2"));   // 0
        System.out.println(s.solve("((1+2)*3-4)*5"));// 35
    }
}