Question Definition
Given a sequence of n integers a1, a2, …, an, a 132 pattern is a subsequence ai, aj, ak such that i < j < k and ai < ak < aj. Design an algorithm that takes a list of n numbers as input and checks whether there is a 132 pattern in the list.
Note: n will be less than 15,000.
Example 1:
Input: [1, 2, 3, 4]
Output: False
Explanation: There is no 132 pattern in the sequence.
Example 2:
Input: [3, 1, 4, 2]
Output: True
Explanation: There is a 132 pattern in the sequence: [1, 4, 2].
Example 3:
Input: [-1, 3, 2, 0]
Output: True
Explanation: There are three 132 patterns in the sequence: [-1, 3, 2], [-1, 3, 0] and [-1, 2, 0].
Java Solution
public boolean find132pattern(int[] nums) {
int third = Integer.MIN_VALUE;
Deque<Integer> s = new ArrayDeque<>();
for (int i = nums.length - 1; i >= 0; --i) {
if (nums[i] < third) return true;
else while (!s.isEmpty() && nums[i] > s.peek()) {
third = s.poll();
}
s.push(nums[i]);
}
return false;
}
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