added new question (Find next greatest letter)to Binary search quest…

…ions

Twopointer/Binary search/744. Find Smallest Letter Greater Than Target.py

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+'''744. Find Smallest Letter Greater Than Target
+
+You are given an array of characters letters that is sorted in non-decreasing order, and a character target. There are at least two different characters in letters.
+
+Return the smallest character in letters that is lexicographically greater than target. If such a character does not exist, return the first character in letters.
+
+ 
+
+Example 1:
+
+Input: letters = ["c","f","j"], target = "a"
+Output: "c"
+Explanation: The smallest character that is lexicographically greater than 'a' in letters is 'c'.
+Example 2:
+
+Input: letters = ["c","f","j"], target = "c"
+Output: "f"
+Explanation: The smallest character that is lexicographically greater than 'c' in letters is 'f'.'''
+
+class Solution:
+    def nextGreatestLetter(self, letters: List[str], target: str) -> str:
+
+        start = 0
+        end = len(letters) - 1
+
+        if target < letters[start] or target >= letters[end]:
+            return letters[start]
+
+        while start <= end:
+            middle = start + (end - start) // 2
+            candidate = letters[middle]
+
+            if target < candidate:
+                end = middle - 1
+
+            if target >= candidate:
+                start = middle + 1
+
+        return letters[start]