200 solved. (#51)

Author: zwfang

README.md

@@ -23,6 +23,7 @@ continually updating 😃.
 * [121. Best Time to Buy and Sell Stock](src/0121_best_time_to_buy_and_sell_stock/maxprofit.go)   *`dynamic programming;`*  *`array`*
 * [122. Best Time to Buy and Sell Stock II](src/0122_best_time_to_buy_and_sell_stock_2/maxprofit.go)   *`greedy;`*  *`array`*
 * [167. Two Sum II - Input array is sorted](./src/0167_two_sum2/two_sum2.go)   *`double index;`*  *`binary search`*
+* [200. Number of Islands](src/0200_number_of_island/number_of_island.go)   *`dfs;`*  *`bfs`*
 * [209. Minimum Size Subarray Sum](./src/0209_minimum_size_subarray_sum/minimum_size_subarray_sum.go)   *`sliding window`*
 * [215. Kth Largest Element in an Array](./src/0215_kth_largest_element_in_an_array/kthleiaa.go)   *`sort`*
 * [283. Move Zeroes(solution1)](./src/0283_move_zeroes/move_zeroes.go)   *`sliding window`*

src/0200_number_of_island/number_of_island.go

@@ -0,0 +1,61 @@
+/*
+200. Number of Islands
+https://leetcode.com/problems/number-of-islands/
+
+Given a 2d grid map of '1's (land) and '0's (water), count the number of islands.
+An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically.
+You may assume all four edges of the grid are all surrounded by water.
+*/
+// time: 2019-01-07
+
+package noi
+
+var (
+	d       = [][]int{{0, 1}, {1, 0}, {0, -1}, {-1, 0}}
+	m, n    int
+	visited [][]bool
+)
+
+// flood-fill
+// time complexity: O(n*m)
+// space complexity: O(n*m)
+func numIslands(grid [][]byte) int {
+	m = len(grid)
+	if m <= 0 {
+		return 0
+	}
+	n = len(grid[0])
+
+	visited = make([][]bool, m)
+	for i := 0; i < m; i++ {
+		visited[i] = make([]bool, n)
+	}
+
+	var res int
+
+	for i := 0; i < m; i++ {
+		for j := 0; j < n; j++ {
+			if grid[i][j] == '1' && !visited[i][j] {
+				res++
+				dfs(grid, i, j)
+			}
+		}
+	}
+	return res
+}
+
+// 从grid[x][y]的位置开始，进行floodfill。
+func dfs(grid [][]byte, x, y int) {
+	visited[x][y] = true
+	for i := 0; i < len(d); i++ {
+		newX := x + d[i][0]
+		newY := y + d[i][1]
+		if inArea(newX, newY) && !visited[newX][newY] && grid[newX][newY] == '1' {
+			dfs(grid, newX, newY)
+		}
+	}
+}
+
+func inArea(x, y int) bool {
+	return x >= 0 && x < m && y >= 0 && y < n
+}

src/0200_number_of_island/number_of_island_test.go

@@ -0,0 +1,23 @@
+package noi
+
+import "testing"
+
+func TestNumIslands(t *testing.T) {
+	grids := [][][]byte{
+		{
+			{'1', '1', '0', '0', '0'},
+			{'1', '1', '0', '0', '0'},
+			{'0', '0', '1', '0', '0'},
+			{'0', '0', '0', '1', '1'},
+		},
+		{},
+	}
+	expected := []int{3, 0}
+
+	for index, grid := range grids {
+		if res := numIslands(grid); res != expected[index] {
+			t.Errorf("expected %d, got %d", expected[index], res)
+		}
+	}
+
+}

src/README.md

@@ -63,6 +63,7 @@
 |0165|[165. Compare Version Numbers](0165_compare_version_numbers/compare_version_numbers.go)|Medium|*`string`*|
 |0167|[Two Sum II - Input array is sorted](./0167_two_sum2/two_sum2.go)|Easy|*`对撞指针(双索引)`*|
 |0198|[House Robber](./0198_house_robber/house_robber.go)|Easy|*`memory search;`* *`dynamic programming`*|
+|0200|[200. Number of Islands](0200_number_of_island/number_of_island.go)|Medium|*`dfs;`* *`bfs`*|
 |0209|[Minimum Size Subarray Sum](./0209_minimum_size_subarray_sum/minimum_size_subarray_sum.go)|Medium|*`sliding window`*|
 |0215|[215. Kth Largest Element in an Array](0215_kth_largest_element_in_an_array/kthleiaa.go)|Medium|*`sort`*|
 |0226|[Invert Binary Tree](./0226_invert_binary_tree/invert_binary_tree.go)|Easy|*`recursion; `* *`binary tree`*|