LeetCode 1901 Find a Peak Element II 二分

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].

You may assume that the entire matrix is surrounded by an outer perimeter with the value -1 in each cell.

You must write an algorithm that runs in \(O(m \log(n))\) or \(O(n \log(m))\) time.

Solution

遍历每一行，然后对每一行进行二分，然后对于周边的四个都进行 \(check\):

点击查看代码

class Solution {
public:
    vector<int> findPeakGrid(vector<vector<int>>& mat) {
        int r=mat.size(), c=mat[0].size();
        for(int i=0;i<r;i++){
            int lft=0,rft=c-1;
            while(lft<=rft){
                int mid = (rft-lft)/2+lft;
                int up = i-1>=0?mat[i-1][mid]:INT_MIN;
                int dw = i+1<r?mat[i+1][mid]:INT_MIN;
                int L = mid-1>=0?mat[i][mid-1]:INT_MIN;
                int R = mid+1<c?mat[i][mid+1]:INT_MIN;
                
                if(mat[i][mid]>up&&mat[i][mid]>dw&&mat[i][mid]>L&&mat[i][mid]>R)return {i,mid};
                else if(L<R)lft=mid+1;
                else rft=mid-1;
            }
        }
        return {-1,-1};
    }
};