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| 1 | +package com.zejian.structures.Stack; |
| 2 | + |
| 3 | +/** |
| 4 | + * Created by zejian on 2016/11/28. |
| 5 | + * Blog : http://blog.csdn.net/javazejian [原文地址,请尊重原创] |
| 6 | + * 中缀转后缀,然后计算后缀表达式的值 |
| 7 | + */ |
| 8 | +public class CalculateExpression { |
| 9 | + |
| 10 | + /** |
| 11 | + * 中缀转后缀 |
| 12 | + * @param expstr 中缀表达式字符串 |
| 13 | + * @return |
| 14 | + */ |
| 15 | + public static String toPostfix(String expstr) |
| 16 | + { |
| 17 | + //创建栈,用于存储运算符 |
| 18 | + SeqStack<String> stack = new SeqStack<>(expstr.length()); |
| 19 | + |
| 20 | + String postfix="";//存储后缀表达式的字符串 |
| 21 | + int i=0; |
| 22 | + while (i<expstr.length()) |
| 23 | + { |
| 24 | + char ch=expstr.charAt(i); |
| 25 | + switch (ch) |
| 26 | + { |
| 27 | + case '+': |
| 28 | + case '-': |
| 29 | + //当栈不为空或者栈顶元素不是左括号时,直接出栈,因此此时只有可能是*/+-四种运算符(根据规则4),否则入栈 |
| 30 | + while (!stack.isEmpty() && !stack.peek().equals("(")) { |
| 31 | + postfix += stack.pop(); |
| 32 | + } |
| 33 | + //入栈 |
| 34 | + stack.push(ch+""); |
| 35 | + i++; |
| 36 | + break; |
| 37 | + case '*': |
| 38 | + case '/': |
| 39 | + //遇到运算符*/ |
| 40 | + while (!stack.isEmpty() && (stack.peek().equals("*") || stack.peek().equals("/"))) { |
| 41 | + postfix += stack.pop(); |
| 42 | + } |
| 43 | + stack.push(ch+""); |
| 44 | + i++; |
| 45 | + break; |
| 46 | + case '(': |
| 47 | + //左括号直接入栈 |
| 48 | + stack.push(ch+""); |
| 49 | + i++; |
| 50 | + break; |
| 51 | + case ')': |
| 52 | + //遇到右括号(规则3) |
| 53 | + String out = stack.pop(); |
| 54 | + while (out!=null && !out.equals("(")) |
| 55 | + { |
| 56 | + postfix += out; |
| 57 | + out = stack.pop(); |
| 58 | + } |
| 59 | + i++; |
| 60 | + break; |
| 61 | + default: |
| 62 | + //操作数直接入栈 |
| 63 | + while (ch>='0' && ch<='9') |
| 64 | + { |
| 65 | + postfix += ch; |
| 66 | + i++; |
| 67 | + if (i<expstr.length()) |
| 68 | + ch=expstr.charAt(i); |
| 69 | + else |
| 70 | + ch='='; |
| 71 | + } |
| 72 | + //分隔符 |
| 73 | + postfix += " "; |
| 74 | + break; |
| 75 | + } |
| 76 | + } |
| 77 | + //最后把所有运算符出栈(规则5) |
| 78 | + while (!stack.isEmpty()) |
| 79 | + postfix += stack.pop(); |
| 80 | + return postfix; |
| 81 | + } |
| 82 | + |
| 83 | + /** |
| 84 | + * 计算后缀表达式的值 |
| 85 | + * @param postfix 传入后缀表达式 |
| 86 | + * @return |
| 87 | + */ |
| 88 | + public static int calculatePostfixValue(String postfix) |
| 89 | + { |
| 90 | + //栈用于存储操作数,协助运算 |
| 91 | + LinkedStack<Integer> stack = new LinkedStack<>(); |
| 92 | + int i=0, result=0; |
| 93 | + while (i<postfix.length()) |
| 94 | + { |
| 95 | + char ch=postfix.charAt(i); |
| 96 | + if (ch>='0' && ch<='9') |
| 97 | + { |
| 98 | + result=0; |
| 99 | + while (ch!=' ') |
| 100 | + { |
| 101 | + //将整数字符转为整数值ch=90 |
| 102 | + result = result*10 + Integer.parseInt(ch+""); |
| 103 | + i++; |
| 104 | + ch = postfix.charAt(i); |
| 105 | + } |
| 106 | + i++; |
| 107 | + stack.push(result);//操作数入栈 |
| 108 | + } |
| 109 | + else |
| 110 | + { //ch 是运算符,出栈栈顶的前两个元素 |
| 111 | + int y= stack.pop(); |
| 112 | + int x= stack.pop(); |
| 113 | + switch (ch) |
| 114 | + { //根据情况进行计算 |
| 115 | + case '+': result=x+y; break; |
| 116 | + case '-': result=x-y; break; |
| 117 | + case '*': result=x*y; break; |
| 118 | + case '/': result=x/y; break; //注意这里并没去判断除数是否为0的情况 |
| 119 | + } |
| 120 | + //将运算结果入栈 |
| 121 | + stack.push(result); |
| 122 | + i++; |
| 123 | + } |
| 124 | + } |
| 125 | + //将最后的结果出栈并返回 |
| 126 | + return stack.pop(); |
| 127 | + } |
| 128 | + //测试 |
| 129 | + public static void main(String args[]) |
| 130 | + { |
| 131 | + String expstr="1+3*(9-2)+90"; |
| 132 | + String postfix = toPostfix(expstr); |
| 133 | + System.out.println("中缀表达式->expstr= "+expstr); |
| 134 | + System.out.println("后缀表达式->postfix= "+postfix); |
| 135 | + System.out.println("计算结果->value= "+calculatePostfixValue(postfix)); |
| 136 | + } |
| 137 | + |
| 138 | +} |
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