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copied!<pre><code>/** * Convert arithmetic from infix to postfix * * @example 1 * ( 12 + 23 ) - ( 4 / 5 ) => 1 12 23 + * 4 5 / - * @author Yong Su */ import java.util.Stack; import java.lang.StringBuilder; class InfixToPostfix { public static void main(String[] args) { String infix = "1 * ( 12 + 23 ) - ( 4 / 5 )"; String postfix = convert(infix); System.out.println(postfix); } /** * Perform the conversion * * @param expression Arithmetic infix format */ public static String convert(String expression) { // Use StringBuilder to append string is faster than // String concatenation StringBuilder sb = new StringBuilder(); // Use a stack to track operations Stack<Character> op = new Stack<Character>(); // Convert expression string to char array char[] chars = expression.toCharArray(); // The length of expression character int N = chars.length; for (int i = 0; i < N; i++) { char ch = chars[i]; if (Character.isDigit(ch)) { // Number, simply append to the result while (Character.isDigit(chars[i])) { sb.append(chars[i++]); } // Use space as delimiter sb.append(' '); } else if (ch == '(') { // Left parenthesis, push to the stack op.push(ch); } else if (ch == ')') { // Right parenthesis, pop and append to the result until meet the left parenthesis while (op.peek() != '(') { sb.append(op.pop()).append(' '); } // Don't forget to pop the left parenthesis out op.pop(); } else if (isOperator(ch)) { // Operator, pop out all higher priority operators first and then push it to the stack while (!op.isEmpty() && priority(op.peek()) >= priority(ch)) { sb.append(op.pop()).append(' '); } op.push(ch); } } // Finally pop out whatever left in the stack and append to the result while(!op.isEmpty()) { sb.append(op.pop()).append(' '); } return sb.toString(); } /** * Check if a character is an operator */ private static boolean isOperator(char ch) { return ch == '^' || ch == '*' || ch == '/' || ch == '+' || ch == '-'; } /** * Define the operator priority */ private static int priority(char operator) { switch (operator) { case '^' : return 3; case '*' : case '/' : return 2; case '+' : case '-' : return 1; } return 0; } } </code></pre>

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