number_of_islands.cpp

puzzles/interviews/training/number_of_islands.cpp

@@ -0,0 +1,115 @@
+/*
+VIM: let b:lcppflags="-std=c++14 -O2 -pthread"
+VIM: let b:wcppflags="/O2 /EHsc /DWIN32"
+VIM-: let b:argv=""
+*/
+#include <iostream>
+#include <exception>
+#include <vector>
+#include <string>
+#include <unordered_map>
+
+/*
+ FB On-site March 
+
+    Q: Find number of Islands. 
+    XXXOO 
+    OOOXX 
+    XXOOX 
+    Return 3 islands. 
+    1 1 1 O O
+    O O O 2 2
+    3 3 O O 2
+*/
+int number_of_islands( const std::vector<std::string>& board )
+{
+    auto n = board.size();
+    auto m = board.front().size();
+    std::unordered_map<int,int> g;
+
+    auto idx = [m](size_t i, size_t j){
+        return i*m+j;
+    };
+
+    auto root = [&g](size_t idx){
+        for (; g[idx] != idx; idx = g[idx] );
+        return idx;
+    };
+
+    for (size_t i = 0; i < n; ++i ){
+        for (size_t j = 0; j < m; ++j){
+            if (board[i][j] == 'x'){
+                if (j>0 && board[i][j-1] == 'x'){
+                    g[idx(i,j)]=g[idx(i,j-1)];
+                    if (i>0 && board[i-1][j] == 'x'){
+                        g[root(idx(i-1,j))]=g[idx(i,j)];
+                    }
+                }
+                else if (i>0 && board[i-1][j] == 'x'){
+                    g[idx(i,j)]=g[idx(i-1,j)];
+                }
+                else {
+                    g[idx(i,j)]=idx(i,j);
+                }
+            }
+        }
+    }
+
+    int count = 0;
+    for ( const auto& node: g){
+        if ( node.first == node.second ){
+            ++count;
+        }
+    }
+
+    return count;
+}
+
+int main ( void )
+{try{
+
+    std::cout << number_of_islands({
+                "xxxoo",
+                "oooxx",
+                "xxoox"
+            }) << std::endl;
+
+    std::cout << number_of_islands({
+                ".xx..",
+                "xx...",
+                "xxxxx"
+            }) << std::endl;
+    std::cout << number_of_islands({
+                "....x",
+                "....x",
+                "xxxxx"
+            }) << std::endl;
+    std::cout << number_of_islands({
+                "xxxxx",
+                "xxxxx",
+                "xxxxx"
+            }) << std::endl;
+    std::cout << number_of_islands({
+                "xxxxx",
+                "....x",
+                "xxx.x",
+                "x...x",
+                "xxxxx"
+            }) << std::endl;
+
+
+    return 0;
+}
+catch ( const std::exception& e )
+{
+    std::cerr << std::endl
+            << "std::exception(\"" << e.what() << "\")." << std::endl;
+    return 2;
+}
+catch ( ... )
+{
+    std::cerr  << std::endl
+            << "unknown exception." << std::endl;
+    return 1;
+}}
+