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].
Solution：Stack
思路：
Time Complexity: O(N) Space Complexity: O(N)
Solution Code：
// 定义顺序：s1, s2, s3
// 大小顺序：s1 < s3 < s2
class Solution {
public boolean find132pattern(int[] nums) {
int s3 = Integer.MIN_VALUE;
Deque<Integer> stack = new ArrayDeque<>();
for(int i = nums.length - 1; i >= 0; i--) {
if(nums[i] < s3) return true;
else {
// 单调递减栈
while(!stack.isEmpty() && nums[i] > stack.peek()) {
s3 = stack.pop();
}
stack.push(nums[i]);
}
}
return false;
}
}
