题解 | #表达式求值#

import java.util.*;


public class Solution {

    private Stack<Character> opStack = new Stack<>();
    private Stack<Integer> numStack = new Stack<>();

    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     * 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) {
        // write code here
        if (s == null || s.length() < 1) {
            throw new IllegalArgumentException("Bad param!");
        }
        s = s.replaceAll(" ", "");
        s += ")";
        char[] arr = s.toCharArray();
        int n = arr.length;
        opStack.push('(');
        for (int i = 0; i < n; i++) {
            char ch = arr[i];
            if (ch == '(') {
                opStack.push('(');
            } else if (ch >= '0' && ch <= '9') {
                int num = ch - '0';
                while (i + 1 < n && arr[i + 1] >= '0' && arr[i + 1] <= '9') {
                    i++;
                    num = num * 10 + (arr[i] - '0');
                }
                numStack.push(num);
            } else if (ch == '+' || ch == '/' || ch == '-' || ch == '*') {
                while (opLevel(ch) <= opLevel(opStack.peek())) {
                    popStackCompute();
                }
                opStack.push(ch);
            } else if (ch == ')') {
                while (opStack.peek() != '(') {
                    popStackCompute();
                }
                opStack.pop();
            }
        }
        return numStack.pop();
    }

    private void popStackCompute() {
        int num2 = numStack.pop();
        int num1 = numStack.pop();
        char op = opStack.pop();
        if (op == '+') {
            numStack.push(num1 + num2);
        } else if (op == '-') {
            numStack.push(num1 - num2);
        } else if (op == '/') {
            numStack.push(num1 / num2);
        } else if (op == '*') {
            numStack.push(num1 * num2);
        }
    }

    private int opLevel(char op) {
        switch (op) {
            case '/':
            case '*':
                return 2;
            case '+':
            case '-':
                return 1;
            case '(':
                return 0;
        }
        return 0;
    }
}