added problem 1970

Author: SHA65536

problems/problem1970/last_cross.go

@@ -0,0 +1,94 @@
+package problem1970
+
+/*
+There is a 1-based binary matrix where 0 represents land and 1 represents water.
+You are given integers row and col representing the number of rows and columns in the matrix, respectively.
+Initially on day 0, the entire matrix is land. However, each day a new cell becomes flooded with water.
+You are given a 1-based 2D array cells, where cells[i] = [ri, ci] represents that
+on the ith day, the cell on the rith row and cith column (1-based coordinates) will be covered with water (i.e., changed to 1).
+You want to find the last day that it is possible to walk from the top to the bottom by only walking on land cells.
+You can start from any cell in the top row and end at any cell in the bottom row.
+You can only travel in the four cardinal directions (left, right, up, and down).
+Return the last day where it is possible to walk from the top to the bottom by only walking on land cells.
+*/
+
+type Coord struct {
+	X, Y int
+}
+
+var Start = Coord{-1, -1}
+var End = Coord{-2, -2}
+
+var Moves = [4][2]int{
+	{1, 0}, {0, 1}, {-1, 0}, {0, -1},
+}
+
+func latestDayToCross(row, col int, cells [][]int) int {
+	var res int
+	// swamp[x][y] is 1 if the cells is land, and 0 if water
+	var swamp = make([][]int, row)
+	// Start with everything as water
+	for i := range swamp {
+		swamp[i] = make([]int, col)
+	}
+
+	// groups[coord] is the parent of coord in the disjoint set
+	var groups = map[Coord]Coord{Start: Start, End: End}
+	// Add the start and end to the group
+	for i := 0; i < col; i++ {
+		groups[Coord{0, i}] = Start
+		groups[Coord{row - 1, i}] = End
+	}
+
+	// Start adding land (instead of removing water)
+	for res = len(cells) - 1; res >= 0; res-- {
+		cx, cy := cells[res][0]-1, cells[res][1]-1
+		// Mark current cells as land
+		swamp[cx][cy] = 1
+		for _, mov := range Moves {
+			// Try to connect all adjacent land tiles
+			nx, ny := cx+mov[0], cy+mov[1]
+			if isInside(row, col, nx, ny) && swamp[nx][ny] == 1 {
+				union(groups, Coord{cx, cy}, Coord{nx, ny})
+			}
+		}
+		// If the parent of start is the parent of end, we are connected
+		if find(groups, Start) == find(groups, End) {
+			return res
+		}
+	}
+	return res
+}
+
+func isInside(row, col, x, y int) bool {
+	return x >= 0 && x < row && y >= 0 && y < col
+}
+
+func find(uf map[Coord]Coord, x Coord) Coord {
+	if uf[x] == x {
+		// If x is the parent of itself, it is the root of the group
+		return uf[x]
+	} else {
+		// If x is not the parent of itself, we call this function again
+		// to find the real parent, and update the map
+		uf[x] = find(uf, uf[x])
+		return uf[x]
+	}
+}
+
+func union(uf map[Coord]Coord, x, y Coord) {
+	var rootx, rooty Coord
+	if _, found := uf[x]; !found {
+		// If this is the first time seeing x, set it as the root of it's group
+		uf[x] = x
+	}
+	if _, found := uf[y]; !found {
+		// If this is the first time seeing y, set it as the root of it's group
+		uf[y] = y
+	}
+	// Finding the roots of x and y
+	rootx = find(uf, x)
+	rooty = find(uf, y)
+	// Setting the root of rootx be rooty effectivly merging the groups
+	uf[rootx] = rooty
+}

problems/problem1970/last_cross_test.go

@@ -0,0 +1,38 @@
+package problem1970
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type TestCase struct {
+	Row, Col int
+	Cells    [][]int
+	Expected int
+}
+
+var TestCases = []TestCase{
+	{2, 2, [][]int{{1, 1}, {2, 1}, {1, 2}, {2, 2}}, 2},
+	{2, 2, [][]int{{1, 1}, {1, 2}, {2, 1}, {2, 2}}, 1},
+	{3, 3, [][]int{{1, 2}, {2, 1}, {3, 3}, {2, 2}, {1, 1}, {1, 3}, {2, 3}, {3, 2}, {3, 1}}, 3},
+}
+
+func TestLastDayWeCanCross(t *testing.T) {
+	assert := assert.New(t)
+
+	for _, tc := range TestCases {
+		want := tc.Expected
+		got := latestDayToCross(tc.Row, tc.Col, tc.Cells)
+		assert.Equal(want, got, fmt.Sprintf("%+v", tc))
+	}
+}
+
+func BenchmarkLastDayWeCanCross(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range TestCases {
+			latestDayToCross(tc.Row, tc.Col, tc.Cells)
+		}
+	}
+}

readme.md

@@ -476,6 +476,7 @@ Each problem is in it's own directory, with test files. There are helper package
 | 1926 | [Nearest Exit from Entrance in Maze](https://leetcode.com/problems/nearest-exit-from-entrance-in-maze) | [My Solution](./problems/problem1926) | ✅ |
 | 1962 | [Remove Stones to Minimize the Total](https://leetcode.com/problems/remove-stones-to-minimize-the-total) | [My Solution](./problems/problem1962) | ✅ |
 | 1964 | [Find the Longest Valid Obstacle Course at Each Position](https://leetcode.com/problems/find-the-longest-valid-obstacle-course-at-each-position) | [My Solution](./problems/problem1964) | ✅ |
+| 1970 | [Last Day Where You Can Still Cross](https://leetcode.com/problems/last-day-where-you-can-still-cross) | [My Solution](./problems/problem1970) | ✅ |
 | 1971 | [Find if Path Exists in Graph](https://leetcode.com/problems/find-if-path-exists-in-graph) | [My Solution](./problems/problem1971) | ✅ |
 | 1996 | [The Number of Weak Characters in the Game](https://leetcode.com/problems/the-number-of-weak-characters-in-the-game) | [My Solution](./problems/problem1996) | ✅ |
 | 2000 | [Reverse Prefix of Word](https://leetcode.com/problems/reverse-prefix-of-word/) | [My Solution](./problems/problem2000) | ✅ |