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Java 将表达式转换为后缀

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将表达式转换为后缀

实例源码：

源代码：

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import java.util.HashMap;

import java.util.Map;

import java.util.Stack;

/*
来 自*
 n  o  w  j  a  v  a . c o m
*/

public class Main {

private static String convertToPostfix(String expr) {

Map<Character, Integer> opPrecedence = new HashMap<>();

opPrecedence.put('+', 0);

opPrecedence.put('-', 0);

opPrecedence.put('*', 1);

opPrecedence.put('/', 1);

Stack<Character> stack = new Stack<>();

String output = "";

int i = 0;

while (i < expr.length()) {

char cur = expr.charAt(i);/*时   代    Java - nowjava.com 提供*/

if (cur == ' ') {

++i;

continue;

}

if (Character.isDigit(cur)) {

String numStr = "";

while (i < expr.length() && Character.isDigit(expr.charAt(i))) {

numStr += expr.charAt(i);

++i;

}

output += numStr;

output += ' ';

continue;

}

if (cur == '(') {

stack.push(cur);

++i;

continue;

}

if (cur == ')') {

while (!stack.empty() && stack.peek() != '(') {

output += stack.pop();

output += ' ';

}

if (stack.empty()) {

throw new IllegalArgumentException("Invalid expression");

}

stack.pop();

++i;

continue;

}

if (cur == '+' || cur == '-' || cur == '*' || cur == '/') {

while (!stack.empty() && opPrecedence.containsKey(stack.peek())

&& opPrecedence.get(stack.peek()) >= opPrecedence.get(cur)) {

output += stack.pop();

output += ' ';

}

stack.push(cur);

++i;

continue;

}

throw new IllegalArgumentException("Invalid expression");

}

while (!stack.empty()) {
/**代码未完, 请加载全部代码(NowJava.com).**/

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编辑于 2020-03-26 09:23:272020-03-26 09:23:27

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