[LeetCode] 334. Increasing Triplet Subsequence

 轉自LeetCode

Given an integer array nums, return true if there exists a triple of indices (i, j, k) such that i < j < k and nums[i] < nums[j] < nums[k]. If no such indices exists, return false.

 

Example 1:

Input: nums = [1,2,3,4,5]
Output: true
Explanation: Any triplet where i < j < k is valid.

Example 2:

Input: nums = [5,4,3,2,1]
Output: false
Explanation: No triplet exists.

Example 3:

Input: nums = [2,1,5,0,4,6]
Output: true
Explanation: The triplet (3, 4, 5) is valid because nums[3] == 0 < nums[4] == 4 < nums[5] == 6.

 

Constraints:

• 1 <= nums.length <= 5 * 105
• -231 <= nums[i] <= 231 - 1

Solution

300. Longest Increasing Subsequence 的衍生題

可以用同一個概念來解，多個檢查條件是 ans.size == 3 的時候，就回傳 true

以及結束的時候，檢查 ans.size == 3，因為有可能最後一個數字才滿足條件

Kotlin ( O(nlogn) )

	class Solution {
	fun increasingTriplet(nums: IntArray): Boolean {
	val ans = arrayListOf<Int>()
	ans.add(nums[0])
	var left = 0
	var right = 0
	var mid = 0
	for(i in 1..nums.lastIndex) {
	when {
	ans.size == 3 -> return true
	nums[i] <= ans[0] -> ans[0] = nums[i]
	nums[i] > ans.last() -> ans.add(nums[i])
	else -> {
	left = 0
	right = ans.size
	while(left < right) {
	mid = left + (right - left) / 2
	if(ans[mid] < nums[i]) {
	left = mid + 1
	} else {
	right = mid
	}
	}
	if(right >= ans.size) {
	ans.add(nums[i])
	} else {
	ans[right] = nums[i]
	}
	}
	}
	}
	return ans.size == 3
	}
	}

view raw increasing_triplet_subsequence.kt hosted with ❤ by GitHub

題目也有問是否可以在 time complexity O(n) 以及 space complexity O(1) 完成

看到一個很厲害的解法 ，其實就是按照題意進行

準備兩個變數，number1 和 number2，兩個的初始值都是 Int_MAX_VALUE

當有個數字小於等於 number1 的時候，不斷更新 number1

當有個數字大於 number1 且小於等於 number2 的時候，不斷更新 number2

這時候，如果還有一個數字大於 number1和 number2，那就找到答案了

kotlin

	class Solution {
	fun increasingTriplet(nums: IntArray): Boolean {
	var number1 = Int.MAX_VALUE
	var number2 = Int.MAX_VALUE
	for(n in nums) {
	when {
	number1 >= n -> number1 = n
	number2 >= n -> number2 = n
	else -> return true
	}
	}
	return false
	}
	}

view raw increasing_triplet_subsequence.kt hosted with ❤ by GitHub