Initial data commit

Task/Parsing-Shunting-yard-algorithm/Java/parsing-shunting-yard-algorithm.java

@@ -0,0 +1,59 @@
+import java.util.Stack;
+
+public class ShuntingYard {
+
+    public static void main(String[] args) {
+        String infix = "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3";
+        System.out.printf("infix:   %s%n", infix);
+        System.out.printf("postfix: %s%n", infixToPostfix(infix));
+    }
+
+    static String infixToPostfix(String infix) {
+        /* To find out the precedence, we take the index of the
+           token in the ops string and divide by 2 (rounding down).
+           This will give us: 0, 0, 1, 1, 2 */
+        final String ops = "-+/*^";
+
+        StringBuilder sb = new StringBuilder();
+        Stack<Integer> s = new Stack<>();
+
+        for (String token : infix.split("\\s")) {
+            if (token.isEmpty())
+                continue;
+            char c = token.charAt(0);
+            int idx = ops.indexOf(c);
+
+            // check for operator
+            if (idx != -1) {
+                if (s.isEmpty())
+                    s.push(idx);
+
+                else {
+                    while (!s.isEmpty()) {
+                        int prec2 = s.peek() / 2;
+                        int prec1 = idx / 2;
+                        if (prec2 > prec1 || (prec2 == prec1 && c != '^'))
+                            sb.append(ops.charAt(s.pop())).append(' ');
+                        else break;
+                    }
+                    s.push(idx);
+                }
+            }
+            else if (c == '(') {
+                s.push(-2); // -2 stands for '('
+            }
+            else if (c == ')') {
+                // until '(' on stack, pop operators.
+                while (s.peek() != -2)
+                    sb.append(ops.charAt(s.pop())).append(' ');
+                s.pop();
+            }
+            else {
+                sb.append(token).append(' ');
+            }
+        }
+        while (!s.isEmpty())
+            sb.append(ops.charAt(s.pop())).append(' ');
+        return sb.toString();
+    }
+}