456. 132 Pattern

Medium (Trung bình) C++ 🔗 Xem trên LeetCode

📋 Đề Bài

Given an array of n integers nums, a 132 pattern is a subsequence of three integers nums[i], nums[j] and nums[k] such that i < j < k and nums[i] < nums[k] < nums[j].

Return true if there is a 132 pattern in nums, otherwise, return false.

 

Example 1:

Input: nums = [1,2,3,4]
Output: false
Explanation: There is no 132 pattern in the sequence.

Example 2:

Input: nums = [3,1,4,2]
Output: true
Explanation: There is a 132 pattern in the sequence: [1, 4, 2].

Example 3:

Input: nums = [-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].

 

Constraints:

  • n == nums.length
  • 1 <= n <= 2 * 105
  • -109 <= nums[i] <= 109

🧠 Thuật Toán & Kỹ Thuật

Stack (Ngăn xếp)
⏱️ Thời gian O(n)
💾 Không gian O(n)

💻 Lời Giải

C++ 0456-132-pattern.cpp 
class Solution {
public:
    bool find132pattern(vector<int>& nums) {
        const int n = nums.size();
        int s3 = INT_MIN;
        stack<int> st;
        
        for (int i = n - 1; i >= 0; --i) {
            if (s3 > nums[i]) {
                return true;
            }
            while (!st.empty() and nums[i] > st.top()) {
                s3 = st.top();
                st.pop();
            }
            st.push(nums[i]);    
        }
        
        return false;
    }
};