ccf05017 - week17 (34.Permutation)

* beomseok - week17 34

* typo: Problem num & url

Author: xfrnk2

LeetCode-Problems/34.Find_First_and_Last_Position_of_Element_in_Sorted_Array/beomseok.md

@@ -0,0 +1,38 @@
+### Problem
+[34. Find First and Last Position of Element in Sorted Array]([https://leetcode.com/problems/search-insert-position](https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array)
+https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array/
+### Solution
+1. Perform two modified binary searches
+2. One that always moves the right boundary leftward upon seeing target (to find the first occurrence).
+3. One that always moves the left boundary rightward upon seeing target (to find the last occurrence).
+4. Call this routine twice—once with findFirst = True, once with findFirst = False.
+5. The two results give [first_index, last_index].
+
+### Code
+```python
+class Solution:
+    def binary_search(self, nums: List[int], target: int, findFirst: bool) -> int:
+
+        start, end = 0, len(nums) - 1
+        result = -1
+
+        while start <= end:
+            mid = (start + end) // 2
+            if nums[mid] == target:
+                result = mid
+                if findFirst:
+                    end = mid - 1
+                else:
+                    start = mid + 1
+            elif nums[mid] < target:
+                start = mid + 1
+            else:
+                end = mid - 1
+
+        return result
+
+    def searchRange(self, nums: List[int], target: int) -> List[int]:
+        first_pos = self.binary_search(nums, target, True)
+        last_pos  = self.binary_search(nums, target, False)
+        return [first_pos, last_pos]
+```