Solve problem #394

Author: rayworks

src/main/java/org/sean/stack/StringDecoder.java

@@ -0,0 +1,73 @@
+package org.sean.stack;
+
+import java.util.Stack;
+
+/***
+ * 394. Decode String
+ */
+public class StringDecoder {
+    private Stack<String> stack = new Stack<>();
+
+    // stack ops involve digits and chars
+    // * [ -> digit : keep pushing in
+    // * ] -> strs = chars * digit, push(strs)
+    // * char -> pushed in
+    public String decodeString(String s) {
+        int len = s.length();
+        int i = 0;
+        StringBuilder numBuilder = new StringBuilder();
+        while (i < len) {
+            char ch = s.charAt(i);
+            if (Character.isDigit(ch)) {
+                numBuilder.append(ch);
+            } else if (ch == '[') {
+                if (numBuilder.length() > 0) {
+                    // int multipler = Integer.parseInt(numBuilder.toString());
+                    stack.push(numBuilder.toString());
+
+                    numBuilder = new StringBuilder();
+                }
+            } else if (ch == ']') {
+                StringBuilder builder = new StringBuilder();
+                String str = stack.pop();
+                int n = Integer.parseInt(stack.pop());
+
+                for (int j = 0; j < n; j++) {
+                    builder.append(str);
+                }
+                if (!stack.isEmpty()) {
+                    String top = stack.peek();
+                    if (Character.isLetter(top.charAt(0))) {
+                        stack.pop();
+                        stack.push(builder.insert(0, top).toString());
+                    } else {
+                        stack.push(builder.toString());
+                    }
+                } else {
+                    stack.push(builder.toString());
+                }
+
+            } else { // letter
+                if (!stack.isEmpty()) {
+                    String top = stack.peek();
+                    if (Character.isDigit(top.substring(0, 1).charAt(0))) {
+                        stack.push(ch + "");
+                    } else {
+                        stack.pop();
+                        stack.push(top + ch);
+                    }
+                } else {
+                    stack.push(ch + "");
+                }
+            }
+            ++i;
+        }
+
+        StringBuilder sb = new StringBuilder();
+        while (!stack.isEmpty()) {
+            sb.append(stack.pop());
+        }
+
+        return sb.toString();
+    }
+}

src/test/java/org/sean/stack/StringDecoderTest.java

@@ -0,0 +1,25 @@
+package org.sean.stack;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class StringDecoderTest {
+    private StringDecoder decoder = new StringDecoder();
+
+    @Test
+    public void testStr() {
+        Assert.assertEquals("accaccacc", decoder.decodeString("3[a2[c]]"));
+        Assert.assertEquals("abcabccdcdcdef", decoder.decodeString("2[abc]3[cd]ef"));
+
+        Assert.assertEquals("aaabFFFFcbFFFFc", decoder.decodeString("3[a]2[b4[F]c]"));
+    }
+
+    @Test
+    public void decodeLongString() {
+        String t = "3[z]2[2[y]pq4[2[jk]e1[f]]]ef";
+        Assert.assertEquals("zzzyypqjkjkefjkjkefjkjkefjkjkefyypqjkjkefjkjkefjkjkefjkjkefef",
+                decoder.decodeString(t));
+    }
+}