Create coloring-a-border.cpp

Author: kamyu104

C++/coloring-a-border.cpp

@@ -0,0 +1,52 @@
+// Time:  O(m * n)
+// Space: O(m + n)
+
+class Solution {
+public:
+    template <typename T>
+    struct PairHash {
+        size_t operator()(const pair<T, T>& p) const {
+            size_t seed = 0;
+            seed ^= std::hash<T>{}(p.first)  + 0x9e3779b9 + (seed<<6) + (seed>>2);
+            seed ^= std::hash<T>{}(p.second) + 0x9e3779b9 + (seed<<6) + (seed>>2);
+            return seed;
+        }
+    };
+
+    vector<vector<int>> colorBorder(vector<vector<int>>& grid, int r0, int c0, int color) {
+        static const vector<pair<int, int>> directions{{0, -1}, {0, 1},
+                                                       {-1, 0}, {1, 0}};
+        vector<pair<int, int>> borders;
+        unordered_set<pair<int, int>, PairHash<int>> lookup{make_pair(r0, c0)};
+	    queue<pair<int, int>> q({make_pair(r0, c0)});
+        while (!q.empty()) {
+            int r, c;
+            tie(r, c) = q.front();  q.pop();
+            bool is_border = false;
+
+            for (const auto& dir : directions) {
+                const auto nr = r + dir.first, nc = c + dir.second;
+                if (!((0 <= nr && nr < grid.size()) &&
+                      (0 <= nc && nc < grid[0].size()) &&
+                      grid[nr][nc] == grid[r][c])) {
+                    is_border = true;
+                    continue;
+                }
+                if (lookup.count(make_pair(nr, nc))) {
+                    continue;
+                }
+                lookup.emplace(nr, nc);
+                q.emplace(nr, nc);
+            }
+
+            if (is_border) {
+                borders.emplace_back(r, c);
+            }
+        }
+
+        for (const auto& border : borders) {
+            grid[border.first][border.second] = color;
+        }
+        return grid;
+    }
+};