Added task 394.

Author: javadev

src/main/java/g0301_0400/s0394_decode_string/Solution.java

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+package g0301_0400.s0394_decode_string;
+
+// #Medium #Top_100_Liked_Questions #String #Stack #Recursion
+
+public class Solution {
+    private int i = 0;
+
+    public String decodeString(String s) {
+        int count = 0;
+        StringBuilder sb = new StringBuilder();
+        while (i < s.length()) {
+            char c = s.charAt(i);
+            i++;
+            if (Character.isLetter(c)) {
+                sb.append(c);
+            } else if (Character.isDigit(c)) {
+                count = count * 10 + Character.getNumericValue(c);
+            } else if (c == ']') {
+                break;
+            } else if (c == '[') {
+                // sub problem
+                String repeat = decodeString(s);
+                while (count > 0) {
+                    sb.append(repeat);
+                    count--;
+                }
+            }
+        }
+        return sb.toString();
+    }
+}

src/main/java/g0301_0400/s0394_decode_string/readme.md

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+394\. Decode String
+
+Medium
+
+Given an encoded string, return its decoded string.
+
+The encoding rule is: `k[encoded_string]`, where the `encoded_string` inside the square brackets is being repeated exactly `k` times. Note that `k` is guaranteed to be a positive integer.
+
+You may assume that the input string is always valid; No extra white spaces, square brackets are well-formed, etc.
+
+Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, `k`. For example, there won't be input like `3a` or `2[4]`.
+
+**Example 1:**
+
+**Input:** s = "3\[a\]2\[bc\]"
+
+**Output:** "aaabcbc" 
+
+**Example 2:**
+
+**Input:** s = "3\[a2\[c\]\]"
+
+**Output:** "accaccacc" 
+
+**Example 3:**
+
+**Input:** s = "2\[abc\]3\[cd\]ef"
+
+**Output:** "abcabccdcdcdef" 
+
+**Example 4:**
+
+**Input:** s = "abc3\[cd\]xyz"
+
+**Output:** "abccdcdcdxyz" 
+
+**Constraints:**
+
+*   `1 <= s.length <= 30`
+*   `s` consists of lowercase English letters, digits, and square brackets `'[]'`.
+*   `s` is guaranteed to be **a valid** input.
+*   All the integers in `s` are in the range `[1, 300]`.

src/test/java/g0301_0400/s0394_decode_string/SolutionTest.java

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+package g0301_0400.s0394_decode_string;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+    @Test
+    void decodeString() {
+        assertThat(new Solution().decodeString("3[a]2[bc]"), equalTo("aaabcbc"));
+    }
+
+    @Test
+    void decodeString2() {
+        assertThat(new Solution().decodeString("3[a2[c]]"), equalTo("accaccacc"));
+    }
+
+    @Test
+    void decodeString3() {
+        assertThat(new Solution().decodeString("2[abc]3[cd]ef"), equalTo("abcabccdcdcdef"));
+    }
+
+    @Test
+    void decodeString4() {
+        assertThat(new Solution().decodeString("abc3[cd]xyz"), equalTo("abccdcdcdxyz"));
+    }
+}