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wuduhren · wuduhren · commit 158fa826795f · 2021-09-10T07:36:08.000+08:00
diff --git a/problems/first-missing-positive.py b/problems/first-missing-positive.py
@@ -0,0 +1,58 @@
+class Solution(object):
+    def firstMissingPositive(self, nums):
+        N = len(nums)
+        
+        if 1 not in nums: return 1
+        
+        for i, num in enumerate(nums):
+            if num<=0 or num>N:
+                nums[i] = 1
+        
+        for i in xrange(N):
+            a = abs(nums[i])
+            
+            if a==N:
+                nums[0] = -abs(nums[0])
+            else:
+                nums[a] = -abs(nums[a])
+        
+        for i in xrange(1, N):
+            if nums[i]>0: return i
+        if nums[0]>0: return N
+        
+        return N+1
+
+"""
+Time: O(N), each num is going to be swap once.
+Space: O(1).
+
+For nums with length N, the answer must be in between 1~N+1.
+
+[1] So for number between 1~N, lets put them in the right place.
+number 1 will be stored at index 0.
+number 2 will be stored at index 1.
+...
+number n will be stored at index n-1.
+
+
+[2] Check if any number are not in place.
+
+[3] If all the number are in place, then the ans must be N+1.
+"""
+class Solution(object):
+    def firstMissingPositive(self, nums):
+        N = len(nums)
+        
+        #[1]
+        for i in xrange(N):
+            while 1<=nums[i]<=N and nums[i]!=i+1:
+                j = nums[i]-1
+                if nums[i]==nums[j]: break #otherwise they are going to keep swapping infinitely, because one of them is not inplace.
+                nums[i], nums[j] = nums[j], nums[i]
+        
+        #[2]
+        for i in xrange(N):
+            if nums[i]!=i+1: return i+1
+        
+        #[3]
+        return N+1
