feat: Add top k frequent words algorithm

dynamic_programming/decode_ways.py

@@ -0,0 +1,47 @@
+"""
+A message containing letters from A-Z is being encoded to numbers using the
+following mapping:
+'A' -> 1
+'B' -> 2
+...
+'Z' -> 26
+Given a non-empty string containing only digits,
+determine the total number of ways to decode it.
+"""
+
+def num_decodings(s: str):
+    """
+    >> num_decodings("12")
+    2
+    >> num_decodings("226")
+    3
+    """
+    if not s or int(s[0]) == 0:
+        return 0
+
+    last = 1
+    second_last = 1
+
+    for i in range(1, len(s)):
+        # 0 is a special digit since it does not
+        # correspond to any alphabet but can be
+        # meaningful if preceeded by 1 or 2
+
+        if s[i] == "0":
+            if s[i-1] in {"1", "2"}:
+                curr = second_last
+            else:
+                return 0
+        elif 11 <= int(s[i-1:i+1]) <= 26:
+            curr = second_last + last
+        else:
+            curr = last
+
+        last, second_last = curr, last
+    return last
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()

hashes/topk.py

@@ -0,0 +1,33 @@
+"""
+Given a non-empty list of words, return the k most frequent elements.
+Your answer should be sorted by frequency from highest to lowest. 
+If two words have the same frequency, then the word with the lower
+alphabetical order comes first.
+"""
+import collections
+
+def topk_frequent(self, words, k):
+    """
+    >> topk_frequent(["i", "love", "leetcode", "i", "love", "coding"], 2)
+    ["i", "love"]
+    """
+    count = collections.Counter(words)
+    freqMap = collections.defaultdict(list)
+
+    for word, freq in count.items():
+        freqMap[freq].append(word)
+
+    freqs = list(freqMap.keys())
+    heapq.heapify(freqs)
+
+    topK = []
+    for freq in heapq.nlargest(k, freqs):
+        topK.extend(sorted(freqMap[freq]))
+        if len(topK) >= k:
+            return topK[:k]
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()