problem: 0744 find smallest letter greater than target

Author: cychiu8

0744 Find Smallest Letter Greater Than Target/0744 Find Smallest Letter Greater Than Target.md

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+# 0744. Find Smallest Letter Greater Than Target
+
+* Difficulty: easy
+* Link: https://leetcode.com/problems/find-smallest-letter-greater-than-target/
+* Topics: Binary-Search
+
+# Clarification
+
+1. Check the inputs and outputs
+    - INPUT:
+        - List[char]
+        - char
+    - OUTPUT:char
+
+# Solution
+
+### Thought Process
+
+- binary search
+- 一開始直接先判斷超出的部分
+- 兩個 pointer
+    - left: 指向陣列中最左邊的 char
+    - right: 指向陣列中最右邊的 char
+- 每次比較 mid = (left+right)/2 index 的 char 與 target
+- 若 mid > target → right = mid -1
+- 若 mid ≤ target → left = mid + 1
+- Implement
+    
+    ```python
+    class Solution:
+        def nextGreatestLetter(self, letters: List[str], target: str) -> str:
+            # if the number is out of bound
+            if target >= letters[-1] or target < letters[0]:
+                return letters[0]
+            
+    				# binary search
+            left = 0
+            right = len(letters)-1
+            while left <= right:
+                mid = (left+right)//2
+                if  target >= letters[mid]:
+                    left = mid+1
+                if target < letters[mid]:
+                    right = mid-1
+                    
+            return letters[left]
+    ```
+    
+
+### Complexity
+
+- Time complexity: log(n)
+- Space complexity: log(1)
+
+# Note
+
+- ****[Python easy solution with detail explanation (modified binary search)](https://leetcode.com/problems/find-smallest-letter-greater-than-target/discuss/1568523/Python-easy-solution-with-detail-explanation-(modified-binary-search))****