Number of Islands
Medium
Array
DFS
BFS
Union Find
Matrix
Given an `m x n` 2D binary grid `grid` which represents a map of `'1'`s (land) and `'0'`s (water), return *the number of islands*. An **island** is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.
Example 1
Input: grid = [["1","1","1","1","0"],["1","1","0","1","0"],["1","1","0","0","0"],["0","0","0","0","0"]]
Output: 1
Example 2
Input: grid = [["1","1","0","0","0"],["1","1","0","0","0"],["0","0","1","0","0"],["0","0","0","1","1"]]
Output: 3

Constraints

  • •m == grid.length
  • •n == grid[i].length
  • •1 <= m, n <= 300
  • •grid[i][j] is '0' or '1'.
Time Complexity
O(m×n)
Space Complexity
O(m×n)
14
Case 1
Input: [["1","1","1","1","0"],["1","1","0","1","0"],["1","1","0","0","0"],["0","0","0","0","0"]]
Expected: 1