Leetcode 334. Increasing Triplet Subsequence

https://leetcode.com/problems/increasing-triplet-subsequence/

Medium

Given an unsorted array return whether an increasing subsequence of length 3 exists or not in the array.

Formally the function should:

Return true if there exists  i, j, k 
such that  arr[i] <  arr[j] <  arr[k] given 0 ≤  i <  j <  k ≤  n-1 else return false.

Note: Your algorithm should run in O(n) time complexity and O(1) space complexity.

Example 1:

Input: [1,2,3,4,5]
Output: true 

Example 2:

Input: [5,4,3,2,1]
Output: false

---

• 比較tricky。利用if else語句從小到大判斷，須要注意用<=判斷，由於對數字都相等的case，返回爲False。
• Concise Java solution with comments. - LeetCode Discuss
  • https://leetcode.com/problems/increasing-triplet-subsequence/discuss/79004/Concise-Java-solution-with-comments.

 1 class Solution:
 2     def increasingTriplet(self, nums: List[int]) -> bool:
 3         min_num, a, b = float("inf"), float("inf"), float("inf")
 4         
 5         for c in nums:
 6             if c <= min_num: # pay attention to equal case
 7                 min_num = c
 8             elif c <= b:
 9                 a, b = min_num, c
10             else: # a < b < c
11                 return True
12         
13         return False

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