Merge pull request #159 from Tr-Jono/master

Add ternary search algo

Author: ionicc

Arrays-searching/src/tenary_search/ternary_search.py

@@ -0,0 +1,40 @@
+"""
+Ternary Search in Python by @Tr-Jono
+
+Algorithm: (list must be sorted)
+1. If list is empty, return False.
+1. Assign m1 and m2 s.t. they are the boundaries when the list is cut into thirds.
+2. If list[m1] or list[m2] is the desired value, return True.
+3. Else, use the same algorithm on:
+     a. The first third of the list, if the desired value is smaller than list[m1], else
+     b. The last third of the list, if the desired value is larger than list[m2], else
+     c. The middle third of the list.
+
+Time Complexity: O(log3 n)
+"""
+
+
+def ternary_search(a, target):
+    """Ternary search if `target` is in `a`."""
+    if not a:
+        return False
+    m1 = len(a) // 3
+    m2 = len(a) - m1 - 1
+    if a[m1] == target or a[m2] == target:
+        return True
+    if target < a[m1]:
+        return ternary_search(a[:m1], target)
+    elif target > a[m2]:
+        return ternary_search(a[m2 + 1:], target)
+    else:
+        return ternary_search(a[m1 + 1:m2], target)
+
+
+def main():
+    print("ternary_search([1, 2, 3], 5):", ternary_search([1, 2, 3], 5))
+    print("ternary_search([3, 5, 7, 9, 11, 13, 15], 5):", ternary_search([3, 5, 7, 9, 11, 13, 15], 5))
+    print("ternary_search([1, 2, 3, 4, 6, 7, 8, 9, 10], 5):", ternary_search([1, 2, 3, 4, 6, 7, 8, 9, 10], 5))
+
+
+if __name__ == "__main__":
+    main()