Create snakes-and-ladders.cpp

C++/snakes-and-ladders.cpp

@@ -0,0 +1,40 @@
+// Time:  O(n^2)
+// Space: O(n^2)
+
+class Solution {
+public:
+    int snakesAndLadders(vector<vector<int>>& board) {
+        const int n = board.size();
+        unordered_map<int, int> lookup;
+        lookup[1] = 0;
+        queue<int> q({1});
+        while (!q.empty()) {
+            const auto s = q.front(); q.pop();
+            if (s == n * n) {
+                return lookup[s];
+            }
+            for (int i = s + 1; i < min(s + 6, n * n) + 1; ++i) {
+                int r, c;
+                tie(r, c) = coordinate(n, i);
+                int s2 = i;
+                if (board[r][c] != -1) {
+                    s2 = board[r][c];
+                }
+                if (!lookup.count(s2)) {
+                    lookup[s2] = lookup[s] + 1;
+                    q.emplace(s2);
+                }
+            }
+        }
+        return -1;
+    }
+
+private:
+    pair<int, int> coordinate(int n, int s) {
+        const int a = (s - 1) / n;
+        const int b = (s - 1) % n;
+        const int r = n - 1 - a;
+        const int c = (r % 2 != n % 2) ? b : n - 1 - b;
+        return {r, c};
+    }
+};