add new solutions

solutions/19/q1970/solution.go

@@ -0,0 +1,67 @@
+package q1970
+
+var dirs = [4][2]int{{0, 1}, {0, -1}, {1, 0}, {-1, 0}}
+
+func isPossibleOnDay(row, col int, cells [][]bool, day int, floods [][]int) bool {
+	for i := range col {
+		cells[0][i] = true
+	}
+	for i := range row - 1 {
+		copy(cells[i+1], cells[0])
+	}
+	for i := range day {
+		cells[floods[i][0]-1][floods[i][1]-1] = false
+	}
+
+	// Use bfs to find a path from top to bottom
+	queue := make([][2]int, 0, col) // {row, col}
+	for x, ok := range cells[0] {
+		if ok {
+			queue = append(queue, [2]int{0, x})
+			cells[0][x] = false
+		}
+	}
+	for ; len(queue) > 0; queue = queue[1:] {
+		r, c := queue[0][0], queue[0][1]
+		for _, d := range dirs {
+			tr, tc := r+d[0], c+d[1]
+			if tr < 0 || tc < 0 || tr >= row || tc >= col {
+				continue
+			}
+			if !cells[tr][tc] {
+				continue
+			}
+			if tr == row-1 {
+				return true // reached bottom
+			}
+			cells[tr][tc] = false
+			queue = append(queue, [2]int{tr, tc})
+		}
+	}
+	return false
+}
+
+func latestDayToCross(row int, col int, cells [][]int) int {
+	walkableBuf := make([]bool, col*row)
+	walkable := make([][]bool, row)
+	for i := range row {
+		walkable[i] = walkableBuf[i*col : (i+1)*col]
+	}
+
+	// Use binary search to find the last day
+	l, r := 1, len(cells)+1
+	for l < r {
+		if l+1 == r {
+			return l
+		}
+		m := (l + r) / 2
+		if isPossibleOnDay(row, col, walkable, m, cells) {
+			l = m
+		} else {
+			r = m
+		}
+	}
+	return 0 // impossible
+}
+
+var _ = latestDayToCross