Java 将表达式转换为后缀
将表达式转换为后缀
实例源码：
源代码：
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import java.util.HashMap;3
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import java.util.Map;5
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import java.util.Stack;7
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/*9
来 自*10
n o w j a v a . c o m11
*/12
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public class Main {14
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private static String convertToPostfix(String expr) {17
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Map<Character, Integer> opPrecedence = new HashMap<>();19
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opPrecedence.put('+', 0);21
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opPrecedence.put('-', 0);23
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opPrecedence.put('*', 1);25
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opPrecedence.put('/', 1);27
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Stack<Character> stack = new Stack<>();30
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String output = "";32
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int i = 0;35
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while (i < expr.length()) {37
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char cur = expr.charAt(i);/*时 代 Java - nowjava.com 提供*/39
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if (cur == ' ') {42
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++i;44
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continue;46
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}48
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if (Character.isDigit(cur)) {51
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String numStr = "";53
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while (i < expr.length() && Character.isDigit(expr.charAt(i))) {56
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numStr += expr.charAt(i);58
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++i;60
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}62
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output += numStr;65
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output += ' ';67
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continue;70
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}72
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if (cur == '(') {75
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stack.push(cur);77
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++i;80
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continue;83
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}85
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if (cur == ')') {88
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while (!stack.empty() && stack.peek() != '(') {90
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output += stack.pop();92
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output += ' ';94
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}96
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if (stack.empty()) {99
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throw new IllegalArgumentException("Invalid expression");101
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}103
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stack.pop();106
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++i;109
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continue;112
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}114
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if (cur == '+' || cur == '-' || cur == '*' || cur == '/') {117
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while (!stack.empty() && opPrecedence.containsKey(stack.peek())119
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&& opPrecedence.get(stack.peek()) >= opPrecedence.get(cur)) {121
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output += stack.pop();123
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output += ' ';125
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}127
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stack.push(cur);130
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++i;133
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continue;136
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}138
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throw new IllegalArgumentException("Invalid expression");141
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}143
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while (!stack.empty()) {146
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