Increasing Triplet Subsequence

描述

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

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

Your function should run in O(n) time complexity and O(1) space complexity.

Examples:

  • Given [1, 2, 3, 4, 5], return true.
  • Given [5, 4, 3, 2, 1], return false.

    分析

扫描一遍数组，用变量x1保存当前最小的值，变量x2保存当前第二小的值。如果右面能碰到一个数大于x2，说明必然存在一个递增的三元组。

代码

  1. // Increasing Triplet Subsequence
  2. // Time complexity: O(n), Space complexity: O(1)
  3. public class Solution {
  4.     public boolean increasingTriplet(int[] nums) {
  5.         int x1 = Integer.MAX_VALUE;
  6.         int x2 = Integer.MAX_VALUE;
  8.         for (int x : nums) {
  9.             if (x <= x1) x1 = x;
  10.             else if (x <= x2) x2 = x;
  11.             else return true;
  12.         }
  13.         return false;
  14.     }
  15. }

相关题目

原文: https://soulmachine.gitbooks.io/algorithm-essentials/content/java/linear-list/array/increasing-triplet-subsequence.html