[bky373] 9th Week Soultions

clone-graph/bky373.java

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+/*
+// Definition for a Node.
+class Node {
+    public int val;
+    public List<Node> neighbors;
+    public Node() {
+        val = 0;
+        neighbors = new ArrayList<Node>();
+    }
+    public Node(int _val) {
+        val = _val;
+        neighbors = new ArrayList<Node>();
+    }
+    public Node(int _val, ArrayList<Node> _neighbors) {
+        val = _val;
+        neighbors = _neighbors;
+    }
+}
+*/
+
+/*
+ * time: O(N+M), where N is the number of nodes and M is the number of edges.
+ * space: O(N), N is the space for the `visited` hash map.
+ */
+class Solution {
+    private Map<Node, Node> visited = new HashMap<>();
+
+    public Node cloneGraph(Node node) {
+        if (node == null) {
+            return node;
+        }
+
+        if (visited.containsKey(node)) {
+            return visited.get(node);
+        }
+
+        Node cloned = new Node(node.val, new ArrayList());
+        visited.put(node, cloned);
+
+        for (Node neighbor : node.neighbors) {
+            cloned.neighbors.add(cloneGraph(neighbor));
+        }
+        return cloned;
+    }
+}

course-schedule/bky373.java

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+/*
+ * time: O(V + E), where V is the number of courses (vertices) and E is the number of prerequisites (edges).
+ * space: O(V + E), where V is for the map and sets and E is for call stacks, which is bounded by the number of courses and prerequisites.
+ */
+class Solution {
+
+    Map<Integer, List<Integer>> courseToPrerequisites = new HashMap<>();
+    Set<Integer> traversing = new HashSet<>();
+    Set<Integer> finished = new HashSet<>();
+
+    public boolean canFinish(int numCourses, int[][] prerequisites) {
+        for (int[] prerequisite : prerequisites) {
+            courseToPrerequisites.computeIfAbsent(prerequisite[0], key -> new ArrayList<>())
+                                 .add(prerequisite[1]);
+        }
+
+        for (int i = 0; i < numCourses; i++) {
+            if (!dfs(i)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    private boolean dfs(int course) {
+        if (traversing.contains(course)) {
+            return false;
+        }
+        if (finished.contains(course)) {
+            return true;
+        }
+
+        traversing.add(course);
+        if (courseToPrerequisites.containsKey(course)) {
+            List<Integer> requisites = courseToPrerequisites.get(course);
+            for (int req : requisites) {
+                if (!dfs(req)) {
+                    return false;
+                }
+            }
+        }
+        traversing.remove(course);
+        finished.add(course);
+        return true;
+    }
+}

design-add-and-search-words-data-structure/bky373.java

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+/*
+ * time:
+ * - add: O(N)
+ * - search: O(N)
+ *  - N is the length of the word.
+ * space: O(M)
+ * - M is the number of nodes.
+ */
+public class WordDictionary {
+
+    private Node root;
+
+    public WordDictionary() {
+        root = new Node();
+    }
+
+    public void addWord(String word) {
+        Node node = root;
+        for (char ch : word.toCharArray()) {
+            if (!node.subNodes.containsKey(ch)) {
+                node.subNodes.put(ch, new Node());
+            }
+            node = node.subNodes.get(ch);
+        }
+        node.isEndOfWord = true;
+    }
+
+    public boolean search(String word) {
+        return helper(root, word);
+    }
+
+    public boolean helper(Node node, String word) {
+        for (int i = 0; i < word.length(); i++) {
+            char ch = word.charAt(i);
+            if (!node.subNodes.containsKey(ch)) {
+                if (ch == '.') {
+                    for (char x : node.subNodes.keySet()) {
+                        Node child = node.subNodes.get(x);
+                        if (helper(child, word.substring(i + 1))) {
+                            return true;
+                        }
+                    }
+                }
+                return false;
+            } else {
+                node = node.subNodes.get(ch);
+            }
+        }
+        return node.isEndOfWord;
+    }
+
+    private class Node {
+
+        Map<Character, Node> subNodes = new HashMap<>();
+        boolean isEndOfWord;
+    }
+
+}

number-of-islands/bky373.java

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+/*
+ * time: O(M * N)
+ * - M is the number of rows
+ * - N is the number of columns
+ * space: O(min(M, N))
+ * - M is the number of rows
+ * - N is the number of columns
+ * in worst case where the grid is filled with lands, the size of queue can grow up to min(𝑀,𝑁).
+ */
+class Solution {
+
+    private char[][] grid;
+
+    public int numIslands(char[][] grid) {
+        this.grid = grid;
+        int numOfIslands = 0;
+
+        for (int i = 0; i < grid.length; i++) {
+            for (int j = 0; j < grid[0].length; j++) {
+                if (grid[i][j] == '1') {
+                    numOfIslands++;
+                    bfs(i, j);
+                }
+            }
+        }
+        return numOfIslands;
+    }
+
+    private void bfs(int i, int j) {
+        Queue<Integer> que = new LinkedList<>();
+        que.add(i);
+        que.add(j);
+        grid[i][j] = 0;
+
+        int[] yDir = {0, -1, 0, 1};
+        int[] xDir = {1, 0, -1, 0};
+
+        while (!que.isEmpty()) {
+            int y = que.poll();
+            int x = que.poll();
+
+            for (int d = 0; d < 4; d++) {
+                int ny = y + yDir[d];
+                int nx = x + xDir[d];
+
+                if (ny < 0 || ny >= grid.length || nx < 0 || nx >= grid[0].length) {
+                    continue;
+                }
+
+                if (grid[ny][nx] == '1') {
+                    grid[ny][nx] = '0';
+                    que.add(ny);
+                    que.add(nx);
+                }
+            }
+        }
+    }
+}

pacific-atlantic-water-flow/bky373.java

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+/*
+ * time: O(M*N)
+ * space: O(M*N)
+ *  - M is the number of rows
+ *  - N is the number of columns
+ */
+class Solution {
+
+    int[][] heights;
+    int rLen;
+    int cLen;
+    int[] rDirs = {0, -1, 0, 1};
+    int[] cDirs = {1, 0, -1, 0};
+
+    public List<List<Integer>> pacificAtlantic(int[][] heights) {
+        this.heights = heights;
+        this.rLen = heights.length;
+        this.cLen = heights[0].length;
+
+        boolean[][] pacific = new boolean[rLen][cLen];
+        boolean[][] atlantic = new boolean[rLen][cLen];
+
+        for (int i = 0; i < rLen; i++) {
+            dfs(i, 0, pacific);
+            dfs(i, cLen - 1, atlantic);
+        }
+
+        for (int i = 0; i < cLen; i++) {
+            dfs(0, i, pacific);
+            dfs(rLen - 1, i, atlantic);
+        }
+
+        List<List<Integer>> both = new ArrayList<>();
+        for (int i = 0; i < rLen; i++) {
+            for (int j = 0; j < cLen; j++) {
+                if (pacific[i][j] && atlantic[i][j]) {
+                    both.add(List.of(i, j));
+                }
+            }
+        }
+        return both;
+    }
+
+    private void dfs(int r, int c, boolean[][] visited) {
+        visited[r][c] = true;
+
+        for (int d = 0; d < 4; d++) {
+            int nr = r + rDirs[d];
+            int nc = c + cDirs[d];
+
+            if (nr < 0 || nr > rLen - 1 || nc < 0 || nc > cLen - 1) {
+                continue;
+            }
+
+            if (visited[nr][nc]) {
+                continue;
+            }
+
+            if (heights[nr][nc] < heights[r][c]) {
+                continue;
+            }
+
+            dfs(nr, nc, visited);
+        }
+    }
+}