Sync LeetCode submission Runtime - 44 ms (40.92%), Memory - 17.9 MB (66.08%)

Author: jimit105

1516-the-k-th-lexicographical-string-of-all-happy-strings-of-length-n/README.md

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+<p>A <strong>happy string</strong> is a string that:</p>
+
+<ul>
+	<li>consists only of letters of the set <code>[&#39;a&#39;, &#39;b&#39;, &#39;c&#39;]</code>.</li>
+	<li><code>s[i] != s[i + 1]</code> for all values of <code>i</code> from <code>1</code> to <code>s.length - 1</code> (string is 1-indexed).</li>
+</ul>
+
+<p>For example, strings <strong>&quot;abc&quot;, &quot;ac&quot;, &quot;b&quot;</strong> and <strong>&quot;abcbabcbcb&quot;</strong> are all happy strings and strings <strong>&quot;aa&quot;, &quot;baa&quot;</strong> and <strong>&quot;ababbc&quot;</strong> are not happy strings.</p>
+
+<p>Given two integers <code>n</code> and <code>k</code>, consider a list of all happy strings of length <code>n</code> sorted in lexicographical order.</p>
+
+<p>Return <em>the kth string</em> of this list or return an <strong>empty string</strong> if there are less than <code>k</code> happy strings of length <code>n</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> n = 1, k = 3
+<strong>Output:</strong> &quot;c&quot;
+<strong>Explanation:</strong> The list [&quot;a&quot;, &quot;b&quot;, &quot;c&quot;] contains all happy strings of length 1. The third string is &quot;c&quot;.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> n = 1, k = 4
+<strong>Output:</strong> &quot;&quot;
+<strong>Explanation:</strong> There are only 3 happy strings of length 1.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> n = 3, k = 9
+<strong>Output:</strong> &quot;cab&quot;
+<strong>Explanation:</strong> There are 12 different happy string of length 3 [&quot;aba&quot;, &quot;abc&quot;, &quot;aca&quot;, &quot;acb&quot;, &quot;bab&quot;, &quot;bac&quot;, &quot;bca&quot;, &quot;bcb&quot;, &quot;cab&quot;, &quot;cac&quot;, &quot;cba&quot;, &quot;cbc&quot;]. You will find the 9<sup>th</sup> string = &quot;cab&quot;
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= n &lt;= 10</code></li>
+	<li><code>1 &lt;= k &lt;= 100</code></li>
+</ul>

1516-the-k-th-lexicographical-string-of-all-happy-strings-of-length-n/solution.py

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+# Approach 1: Backtracking
+
+# Time: O(n * 2^n)
+# Space: O(2^n)
+
+class Solution:
+    def getHappyString(self, n: int, k: int) -> str:
+        current_str = ''
+        happy_strings = []
+
+        self.generate_happy_strings(n, current_str, happy_strings)
+
+        if len(happy_strings) < k:
+            return ''
+
+        happy_strings.sort()
+        return happy_strings[k - 1]
+
+    def generate_happy_strings(self, n, current_str, happy_strings):
+        if len(current_str) == n:
+            happy_strings.append(current_str)
+            return
+
+        for current_char in ['a', 'b', 'c']:
+            if len(current_str) > 0 and current_str[-1] == current_char:
+                continue
+
+            self.generate_happy_strings(n, current_str + current_char, happy_strings)
+