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

---

• 比较tricky。利用if else语句从小到大判断，需要注意用<=判断，因为对数字都相等的case，返回为False。
• Concise Java solution with comments. - LeetCode Discuss
  • https://leetcode.com/problems/increasing-triplet-subsequence/discuss/79004/Concise-Java-solution-with-comments.

class Solution:
    def increasingTriplet(self, nums: List[int]) -> bool:
        min_num, a, b = float("inf"), float("inf"), float("inf")
        
        for c in nums:
            if c <= min_num: # pay attention to equal case
                min_num = c
            elif c <= b:
                a, b = min_num, c
            else: # a < b < c
                return True
        
        return False