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:
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].
class Solution {
public:
bool find132pattern(vector<int>& nums) {
int s = INT_MIN;
stack<int> t;
for( int i = nums.size()-1; i >= 0; i -- ){
if( nums[i] < s ) return true;
else while( !t.empty() && nums[i] > t.top() ){
s = t.top(); t.pop();
}
t.push(nums[i]);
}
return false;
}
};