LeetCode 200. Number of Islands
LeetCode 200. Number of Islands
LeetCode 200. Number of Islands
LINK: https://leetcode.com/problems/number-of-islands/
Matter
You are given a 2D grid consisting of ‘1’ (land) and ‘0’ (water).
Count how many separate islands exist.
An island is formed by connecting adjacent land cells horizontally or vertically.
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
[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
1 <= m, n <= 300
grid[i][j] is ‘0’ or ‘1’
Problem analysis
This is a classic connected-components problem on a grid.
Each island is a group of connected ‘1’ cells.
The idea is:
- Traverse the grid
- When a ‘1’ is found → start a DFS/BFS
- Mark all connected land as visited
- Increase island count
Solution
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class Solution {
public:
int numIslands(std::vector<std::vector<char>>& grid)
{
int i32Count = 0;
do
{
if(!grid.size())
break;
int i32RowSize = grid.size();
for(int i32Row = 0; i32Row < i32RowSize; ++i32Row)
{
std::vector<char>& vctRow = grid[i32Row];
int i32ColSize = vctRow.size();
for(int i32Col = 0; i32Col < i32ColSize; ++i32Col)
{
if(vctRow[i32Col] == '1')
{
CheckAdjacent(grid, i32Row, i32Col);
++i32Count;
}
}
}
}
while (false);
return i32Count;
}
private:
bool CheckAdjacent(std::vector<std::vector<char>>& grid, int i32Row, int i32Col)
{
bool bReturn = false;
do
{
int i32RowSize = grid.size();
int i32ColSize = grid[i32Row].size();
auto IsValid = [&i32RowSize, &i32ColSize](int i32Row, int i32Col) -> bool
{
return i32Row >= 0 && i32Row < i32RowSize &&
i32Col >= 0 && i32Col < i32ColSize;
};
std::vector<std::pair<int, int>> vctland;
vctland.push_back({i32Row, i32Col});
while(!vctland.empty())
{
std::pair<int, int> land = vctland.back();
vctland.pop_back();
int i32Row = land.first;
int i32Col = land.second;
char* pI8Cur = &grid[i32Row][i32Col];
if(IsValid(i32Row, i32Col) && *pI8Cur == '1')
{
*pI8Cur = '0';
if(IsValid(i32Row, i32Col + 1) && *(pI8Cur + 1) == '1')
vctland.push_back({i32Row, i32Col + 1});
if(IsValid(i32Row, i32Col - 1) && *(pI8Cur - 1) == '1')
vctland.push_back({i32Row, i32Col - 1});
if(IsValid(i32Row - 1, i32Col) && grid[i32Row - 1][i32Col] == '1')
vctland.push_back({i32Row - 1, i32Col});
if(IsValid(i32Row + 1, i32Col) && grid[i32Row + 1][i32Col] == '1')
vctland.push_back({i32Row + 1, i32Col});
}
}
bReturn = true;
}
while (false);
return bReturn;
}
};
Little bit more faster:
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class Solution {
public:
int numIslands(std::vector<std::vector<char>>& grid)
{
int i32Count = 0;
do
{
if(!grid.size())
break;
int i32RowSize = grid.size();
for(int i32Row = 0; i32Row < i32RowSize; ++i32Row)
{
std::vector<char>& vctRow = grid[i32Row];
int i32ColSize = vctRow.size();
for(int i32Col = 0; i32Col < i32ColSize; ++i32Col)
{
if(vctRow[i32Col] == '1')
{
CheckAdjacent(grid, i32Row, i32Col);
++i32Count;
}
}
}
}
while (false);
return i32Count;
}
private:
bool CheckAdjacent(std::vector<std::vector<char>>& grid, int i32Row, int i32Col)
{
bool bReturn = false;
do
{
int i32RowSize = grid.size();
int i32ColSize = grid[i32Row].size();
std::vector<std::pair<int, int>> vctland;
vctland.push_back({i32Row, i32Col});
while(!vctland.empty())
{
std::pair<int, int> land = vctland.back();
vctland.pop_back();
int i32Row = land.first;
int i32Col = land.second;
char* pI8Cur = &grid[i32Row][i32Col];
if(i32Row >= 0 && i32Row < i32RowSize &&
i32Col >= 0 && i32Col < i32ColSize &&
*pI8Cur == '1')
{
*pI8Cur = '0';
if(i32Col + 1 < i32ColSize && *(pI8Cur + 1) == '1')
vctland.push_back({i32Row, i32Col + 1});
if(i32Col > 0 && *(pI8Cur - 1) == '1')
vctland.push_back({i32Row, i32Col - 1});
if(i32Row > 0 && grid[i32Row - 1][i32Col] == '1')
vctland.push_back({i32Row - 1, i32Col});
if(i32Row + 1 < i32RowSize && grid[i32Row + 1][i32Col] == '1')
vctland.push_back({i32Row + 1, i32Col});
}
}
bReturn = true;
}
while (false);
return bReturn;
}
};
Possible, But somtimes stack-overflow issue:
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class Solution {
public:
int numIslands(std::vector<std::vector<char>>& grid)
{
int i32Count = 0;
do
{
if(!grid.size())
break;
int i32RowSize = grid.size();
for(int i32Row = 0; i32Row < i32RowSize; ++i32Row)
{
std::vector<char>& vctRow = grid[i32Row];
int i32ColSize = vctRow.size();
for(int i32Col = 0; i32Col < i32ColSize; ++i32Col)
{
if(vctRow[i32Col] == '1')
{
CheckAdjacent(grid, i32Row, i32Col);
++i32Count;
}
}
}
}
while (false);
return i32Count;
}
private:
bool CheckAdjacent(std::vector<std::vector<char>>& grid, int i32Row, int i32Col)
{
bool bReturn = false;
do
{
if(i32Row < 0 ||
i32Row >= grid.size() ||
i32Col < 0 ||
i32Col >= grid[i32Row].size())
break;
if(grid[i32Row][i32Col] != '1')
break;
grid[i32Row][i32Col] = '0';
CheckAdjacent(grid, i32Row - 1, i32Col);
CheckAdjacent(grid, i32Row + 1, i32Col);
CheckAdjacent(grid, i32Row, i32Col - 1);
CheckAdjacent(grid, i32Row, i32Col + 1);
bReturn = true;
}
while (false);
return bReturn;
}
};
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