added First Missing Positive (hard)

Author: minhyeong-joe

Hard/FirstMissingPositive/README.md

@@ -0,0 +1,26 @@
+
+# First Missing Positive
+[Leetcode Link](https://leetcode.com/problems/first-missing-positive/)
+
+## Problem:
+
+Given an unsorted integer array, find the smallest missing positive integer.
+
+## Example:
+
+```
+Input: [1,2,0]
+Output: 3
+```
+```
+Input: [3,4,-1,1]
+Output: 2
+```
+```
+Input: [7,8,9,11,12]
+Output: 1
+```
+
+## Note:
+
+Your algorithm should run in **O(n) time** and uses **constant extra space O(1)**.

Hard/FirstMissingPositive/solution.py

@@ -0,0 +1,28 @@
+from typing import List
+
+class Solution:
+ def firstMissingPositive(self, nums: List[int]) -> int:
+ # mark negatives and zeros with number larger than len(nums)
+ for i in range(len(nums)):
+ if nums[i] <= 0:
+ nums[i] = len(nums)+1
+ print("after marking useless numbers:", nums)
+ # use negative to mark positive integer has appeared
+ for i in range(len(nums)):
+ absolute = abs(nums[i])
+ if absolute <= len(nums):
+ # in case of duplicate (to prevent more than one negation)
+ nums[absolute-1] = -nums[absolute-1] if nums[absolute-1] > 0 else nums[absolute-1]
+ print("After marking positions:", nums)
+ # first positive position + 1 is the first missing positive, if none, then len(nums) + 1 is the first missing
+ for i in range(len(nums)):
+ if nums[i] > 0:
+ return i+1
+ return len(nums)+1
+
+
+# test driver
+sol = Solution()
+input = [4, 0, 1, 2, 0]
+print("Input:", input)
+print("Output:", sol.firstMissingPositive(input))

README.md

@@ -73,6 +73,7 @@ Languages used: Java and Python
  - [Maximum Score Words Formed by Letters](Hard/MaximumScoreWords)
  - [Reducing Dishes](Hard/ReducingDishes)
  - [Longest Consecutive Sequence](Hard/LongestConsecutiveSequence)
+ - [First Missing Positive](Hard/FirstMissingPositive)
 
 ---