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+// 909. Snakes and Ladders
+#include "leetcode.h"
+
+/*
+ * you are given an 'n x n' integer matrix 'board' where the cells are all
+ * labeled from 1 to 'n^2' in 'boustropheden style' starting from bottom left of
+ * the board, and alternating direction each row. a board square on row 'r' and
+ * column 'c' has a snake of a ladder if 'board[r][c] != -1'. the destination of
+ * that snake or ladder is 'board[r][c]'. squares 1 and 'n^2' are not the
+ * starting points of any snake or ladder. note that you only take a snake or a
+ * ladder at most once per dice roll. if the destination to a snake or ladder is
+ * the start of another snake or ladder, you do not follow the subsequent snake
+ * or ladder.
+ */
+
+struct queue {
+ int num;
+ int moves;
+};
+
+void coordinates(int num, int n, int *row, int *col) {
+ int r = (num - 1) / n, c = (num - 1) % n;
+ if (r % 2 == 1)
+ c = n - 1 - c;
+ *row = n - 1 - r;
+ *col = c;
+}
+
+int snakesAndLadders(int **board, int boardSize, int *boardColSize) {
+ int n = boardSize * boardSize, front = 0, rear = 0;
+ bool *vis = (bool *)calloc(n + 1, sizeof(bool));
+ struct queue *q = (struct queue *)malloc(n * sizeof(struct queue));
+ q[rear].num = 1;
+ q[rear++].moves = 0;
+ vis[1] = true;
+ while (front < rear) {
+ struct queue curr = q[front++];
+ if (curr.num == n) {
+ free(vis);
+ free(q);
+ return curr.moves;
+ }
+ for (int i = 1; i <= 6; i++) {
+ int next = curr.num + 1;
+ if (next > n)
+ continue;
+ int row, col;
+ coordinates(next, boardSize, &row, &col);
+ if (board[row][col] != -1)
+ next = board[row][col];
+ if (next == n) {
+ free(vis);
+ free(q);
+ return curr.moves + 1;
+ }
+ if (!vis[next]) {
+ vis[next] = true;
+ q[rear].num = next;
+ q[rear++].moves = curr.moves + 1;
+ }
+ }
+ }
+ free(vis);
+ free(q);
+ return -1;
+}
+
+int main() {
+ int b1i[6][6] = {{-1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1},
+ {-1, -1, -1, -1, -1, -1}, {-1, 35, -1, -1, 13, -1},
+ {-1, -1, -1, -1, -1, -1}, {-1, 15, -1, -1, -1, -1}};
+ int b2i[2][2] = {{-1, -1}, {-1, 3}};
+ struct two_d_arr b1, b2;
+ two_d_arr_init(&b1, ARRAY_SIZE(b1i), ARRAY_SIZE(b1i[0]), b1i);
+ two_d_arr_init(&b2, ARRAY_SIZE(b2i), ARRAY_SIZE(b2i[0]), b2i);
+ printf("%d\n",
+ snakesAndLadders(b1.arr, b1.row_size, b1.col_size)); // expect: 4
+ printf("%d\n",
+ snakesAndLadders(b2.arr, b2.row_size, b2.col_size)); // expect: 1
+ two_d_arr_free(&b1);
+ two_d_arr_free(&b2);
+}