力扣：736. Lisp 语法解析

模拟题，有机会在做一遍，现在是CV看题解

import com.sun.javafx.collections.MappingChange;

import java.util.*;

/**
 * @author xnl
 * @Description:
 * @date: 2022/7/6   22:05
 */
public class Solution {
    public static void main(String[] args) {
        Solution solution = new Solution();
        String str = "(let x 2 (mult x (let x 3 y 4 (add x y))))";
        System.out.println(solution.evaluate(str));
    }

    Map<String, Deque<Integer>> scope = new HashMap<>();
    int start = 0;

    public int evaluate(String expression) {
        return innerEvaluate(expression);
    }

    private int innerEvaluate(String expression){
        if (expression.charAt(start) != '('){
            // 不等于括号的话，那就是可能是整数获取字母
            if (Character.isLowerCase(expression.charAt(start))){
                String var = parseVar(expression);
                return scope.get(var).peek();
            }
            return parseInt(expression);
        }

        int res;
        start++; // 移除左括号
        if (expression.charAt(start) == 'l'){
            start += 4;
            List<String> vars = new ArrayList<>();
            while (true){
                if (!Character.isLowerCase(expression.charAt(start))){
                    res = innerEvaluate(expression);
                    break;
                }

                String var = parseVar(expression);
                if (expression.charAt(start) == ')'){
                    res = scope.get(var).peek();
                    break;
                }
                vars.add(var);
                start++; // 移除空格
                int e = innerEvaluate(expression);
                scope.putIfAbsent(var, new ArrayDeque<Integer>());
                scope.get(var).push(e);
                start++; // 移除空格
            }
            for (String var : vars) {
                scope.get(var).pop(); // 清除当前作用域的变量
            }
        } else if (expression.charAt(start) == 'a'){
            start += 4;
            int e1 = innerEvaluate(expression);
            start++;
            int e2 = innerEvaluate(expression);
            res = e2 + e1;
        } else {
            start += 5;
            int e1 = innerEvaluate(expression);
            start++;
            int e2 = innerEvaluate(expression);
            res = e1 * e2;
        }
        start++;
        return res;
    }

    /**
     * 解析整数
     * @param expression
     * @return
     */
    private int parseInt(String expression){
        int n = expression.length();
        int res = 0, sign = 1;
        if (expression.charAt(start) == '-'){
            sign = -1;
            start++;
        }
        while (start < n && Character.isDigit(expression.charAt(start))){
            res = res * 10 + (expression.charAt(start) - '0');
            start++;
        }
        return sign * res;
    }

    /**
     * 解析变量
     * @return
     */
    private String parseVar(String expression){
        int n = expression.length();
        StringBuffer sb = new StringBuffer();
        while (start < n && expression.charAt(start) != ' ' && expression.charAt(start) != ')'){
            sb.append(expression.charAt(start));
            start++;
        }
        return sb.toString();
    }
}