infix to postfix

Author: realDuYuanChao

DataStructures/Stacks/InfixToPostfix.java

@@ -0,0 +1,55 @@
+package DataStructures.Stacks;
+
+import java.util.Stack;
+
+public class InfixToPostfix {
+  public static void main(String[] args) throws Exception {
+    assert "32+".equals(infix2PostFix("3+2"));
+    assert "123++".equals(infix2PostFix("1+(2+3)"));
+    assert "34+5*6-".equals(infix2PostFix("(3+4)*5-6"));
+  }
+
+  public static String infix2PostFix(String infixExpression) throws Exception {
+    if (!BalancedBrackets.isBalanced(infixExpression)) {
+      throw new Exception("invalid expression");
+    }
+    StringBuilder output = new StringBuilder();
+    Stack<Character> stack = new Stack<>();
+    for (char element : infixExpression.toCharArray()) {
+      if (Character.isLetterOrDigit(element)) {
+        output.append(element);
+      } else if (element == '(') {
+        stack.push(element);
+      } else if (element == ')') {
+        while (!stack.isEmpty() && stack.peek() != '(') {
+          output.append(stack.pop());
+        }
+        stack.pop();
+      } else {
+        while (!stack.isEmpty() && precedence(element) <= precedence(stack.peek())) {
+          output.append(stack.pop());
+        }
+        stack.push(element);
+      }
+    }
+    while (!stack.isEmpty()) {
+      output.append(stack.pop());
+    }
+    return output.toString();
+  }
+
+  private static int precedence(char operator) {
+    switch (operator) {
+      case '+':
+      case '-':
+        return 0;
+      case '*':
+      case '/':
+        return 1;
+      case '^':
+        return 2;
+      default:
+        return -1;
+    }
+  }
+}