Leetcode #1901: Find a Peak Element II
In this guide, we solve Leetcode #1901 Find a Peak Element II in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
A peak element in a 2D grid is an element that is strictly greater than all of its adjacent neighbors to the left, right, top, and bottom. Given a 0-indexed m x n matrix mat where no two adjacent cells are equal, find any peak element mat[i][j] and return the length 2 array [i,j].
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Array, Binary Search, Matrix
Intuition
The problem structure suggests a monotonic decision, which makes binary search a natural fit.
By halving the search space each step, we reach the answer efficiently.
Approach
Search either directly on a sorted array or on the answer space using a check function.
Each check is fast, and the logarithmic search keeps the overall runtime low.
Steps:
- Define the search bounds.
- Check the mid point condition.
- Narrow the bounds until convergence.
Example
Input: mat = [[1,4],[3,2]]
Output: [0,1]
Explanation: Both 3 and 4 are peak elements so [1,0] and [0,1] are both acceptable answers.
Python Solution
class Solution:
def findPeakGrid(self, mat: List[List[int]]) -> List[int]:
l, r = 0, len(mat) - 1
while l < r:
mid = (l + r) >> 1
j = mat[mid].index(max(mat[mid]))
if mat[mid][j] > mat[mid + 1][j]:
r = mid
else:
l = mid + 1
return [l, mat[l].index(max(mat[l]))]
Complexity
The time complexity is , where and are the number of rows and columns of the matrix, respectively. The space complexity is .
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.