双栈法的两个典型例子

书中双栈法的两个典型例子

基本步骤：创建运算符栈和操作数栈，遇到操作数和运算符分别入栈，忽略 “（”，遇到 “）”则弹出运算符和操作数进行运算，得到结果推入运算数栈

• 表达式求值

import java.util.Scanner;
import java.util.Stack;

public class Expr {
    
    
	//简单起见，输入不省略任何符号
	public static void main(String[] args) {
    
    
		Scanner input = new Scanner(System.in);
		String s = input.next();
		Stack<Character> op = new Stack<>();
		Stack<Double> num = new Stack<>();
		for(int i = 0; i < s.length(); i++) {
    
    
			char ch = s.charAt(i);
			if(ch >= '0' && ch <= '9')
				num.push(Double.parseDouble(ch + ""));
			else if(ch == '+' || ch == '-' || ch == '*' || ch == '/')
				op.push(ch);
			else if(ch == ')') {
    
    
				double op2 = num.pop();
				double op1 = num.pop();
				char opr = op.pop();
				double result = 0;
				switch(opr) {
    
    
				case '+': result = op1 + op2;break;
				case '-': result = op1 - op2;break;
				case '*': result = op1 * op2;break;
				case '/': result = op1 / op2;break;
				}
				num.push(result);
			}
		}
		while(!op.empty()) {
    
    
			double op2 = num.pop();
			double op1 = num.pop();
			char opr = op.pop();
			double result = 0;
			switch(opr) {
    
    
			case '+': result = op1 + op2;break;
			case '-': result = op1 - op2;break;
			case '*': result = op1 * op2;break;
			case '/': result = op1 / op2;break;
			}
			num.push(result);
		}
		System.out.println(num.pop());
	}
}

• 补齐左括号

import java.util.Scanner;
import java.util.Stack;
//1+2)*3-4)*5-6)))
public class Fix {
    
    
	public static void main(String[] args) {
    
    
		Scanner input = new Scanner(System.in);
		String s = input.next();
		Stack<String> op = new Stack<>();
		Stack<String> expr = new Stack<>();
		for(int i = 0; i < s.length(); i++) {
    
    
			char ch = s.charAt(i);
			if(ch >= '0' && ch <= '9')
				expr.push(ch + "");
			else if(ch != ')')
				op.push(ch + "");
			else {
    
    
				String opchar = op.pop();
				String op2 = expr.pop();
				String op1 = expr.pop();
				String temp = "(" + op1 + opchar + op2 + ")";
				expr.push(temp);
			}
		}
		while(!op.empty()) {
    
    
			String opchar = op.pop();
			String op2 = expr.pop();
			String op1 = expr.pop();
			String temp = "(" + op1 + opchar + op2 + ")";
			expr.push(temp);
		}
		while(!expr.empty())
			System.out.println(expr.pop());
	}
}

记录一下，便于回忆