[forest000014] Week 14

binary-tree-level-order-traversal/forest000014.java

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+/*
+# Time Complexity: O(n)
+# Space Complexity: O(n)
+  - 재귀 호출 함수의 파라미터 root, depth, ans
+*/
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> ans = new ArrayList<>();
+
+        dfs(root, 0, ans);
+
+        return ans;
+    }
+
+    private void dfs(TreeNode root, int depth, List<List<Integer>> ans) {
+        if (root == null) return;
+
+        if (ans.size() == depth) {
+            ans.add(new ArrayList<>());
+        }
+        ans.get(depth).add(root.val);
+
+        dfs(root.left, depth + 1, ans);
+        dfs(root.right, depth + 1, ans);
+    }
+}

counting-bits/forest000014.java

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+/*
+# Time Complexity: O(n)
+# Space Complexity: O(1)
+*/
+class Solution {
+    public int[] countBits(int n) {
+        int[] ans = new int[n + 1];
+
+        for (int i = 0; i <= n; i++) {
+            int x = i;
+            while (x > 0) {
+                ans[i] += (x & 1);
+                x >>= 1;
+            }
+        }
+
+        return ans;
+    }
+}

house-robber-ii/forest000014.java

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+/*
+# Time Complexity: O(n)
+# Space Complexity: O(n)
+*/
+class Solution {
+    public int rob(int[] nums) {
+        int n = nums.length;
+        if (n == 1) return nums[0];
+
+        int[][][] dp = new int[n][2][2];
+
+        dp[0][0][0] = 0;
+        dp[0][0][1] = 0;
+        dp[0][1][0] = 0; //
+        dp[0][1][1] = nums[0];
+        for (int i = 1; i < n - 1; i++) {
+            dp[i][0][0] = Math.max(dp[i - 1][0][0], dp[i - 1][1][0]);
+            dp[i][0][1] = Math.max(dp[i - 1][0][1], dp[i - 1][1][1]);
+            dp[i][1][0] = dp[i - 1][0][0] + nums[i];
+            dp[i][1][1] = dp[i - 1][0][1] + nums[i];
+        }
+
+        dp[n - 1][0][0] = Math.max(dp[n - 2][0][0], dp[n - 2][1][0]);
+        dp[n - 1][0][1] = Math.max(dp[n - 2][0][1], dp[n - 2][1][1]);
+        dp[n - 1][1][0] = dp[n - 2][0][0] + nums[n - 1];
+        dp[n - 1][1][1] = -1;
+
+        return Math.max(Math.max(dp[n - 1][0][0], dp[n - 1][0][1]), dp[n - 1][1][0]);
+    }
+}

meeting-rooms-ii/forest000014.java

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+/*
+# Time Complexity: O(nlogn)
+# Space Complexity: O(n)
+*/
+
+class Solution {
+
+    private class Info {
+        int kind; // start = 0, end = 1
+        int time;
+
+        Info(int kind, int time) {
+            this.kind = kind;
+            this.time = time;
+        }
+    }
+    public int minMeetingRooms(int[][] intervals) {
+        int n = intervals.length;
+        List<Info> infos = new ArrayList<>();
+        for (int i = 0; i < n; i++) {
+            infos.add(new Info(0, intervals[i][0]));
+            infos.add(new Info(1, intervals[i][1]));
+        }
+
+        Collections.sort(infos, (o1, o2) -> {
+            if (o1.time == o2.time) {
+                return Integer.compare(o2.kind, o1.kind);
+            }
+            return Integer.compare(o1.time, o2.time);
+        });
+
+        int cnt = 0;
+        int ans = 0;
+        for (Info info : infos) {
+            if (info.kind == 0) cnt++;
+            else cnt--;
+            ans = Math.max(ans, cnt);
+        }
+
+        return ans;
+    }
+}

subtree-of-another-tree/forest000014.java

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+/*
+# Time Complexity: O(n)
+# Space Complexity: O(n)
+  - 재귀 호출 파라미터 O(n) 사이즈 공간 필요
+*/
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    public boolean isSubtree(TreeNode root, TreeNode subRoot) {
+        return dfs(root, subRoot);
+    }
+
+    private boolean dfs(TreeNode root, TreeNode subRoot) {
+        if (root == null && subRoot == null) return true;
+        if (root == null || subRoot == null) return false;
+
+        if (root.val == subRoot.val) {
+            if (compareTrees(root, subRoot)) return true;
+        }
+
+        if (dfs(root.left, subRoot)) return true;
+        if (dfs(root.right, subRoot)) return true;
+
+        return false;
+    }
+
+    private boolean compareTrees(TreeNode root1, TreeNode root2) {
+        if (root1 == null && root2 == null) return true;
+        if (root1 == null || root2 == null) return false;
+        if (root1.val != root2.val) return false;
+
+        return compareTrees(root1.left, root2.left) && compareTrees(root1.right, root2.right);
+    }
+}

word-search-ii/forest000014.java

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+/*
+# Time Complexity: O(w + m * n * 4^10)
+  - trie에 word 하나(최대 길이 10)를 삽입하는 데에는 O(10) = O(1) 이므로, trie 전체를 생성하는 데에는 O(w) (w는 words의 length)
+  - dfs 탐색을 하면서, 모든 경로를 탐색 (최대 depth는 10)
+# Space Complexity: O(w)
+  - trie를 생성하면, 최대 10글자 * w개 문자열 = O(10w) = O(w)
+*/
+class Solution {
+
+    private class Trie {
+        char val;
+        boolean ends;
+        Map<Character, Trie> children;
+
+        Trie(char val) {
+            this.val = val;
+            this.children = new HashMap<>();
+        }
+    }
+    public List<String> findWords(char[][] board, String[] words) {
+        // Trie 생성 및 세팅
+        Trie root = new Trie('.');
+        for (String word : words) {
+            Trie curr = root;
+            for (int i = 0; i < word.length(); i++) {
+                char ch = word.charAt(i);
+                curr.children.putIfAbsent(ch, new Trie(ch));
+                curr = curr.children.get(ch);
+            }
+            curr.ends = true;
+        }
+
+        // trie와 dfs를 사용하여, 단어가 존재하는지 확인
+        int m = board.length;
+        int n = board[0].length;
+        boolean[][] visited = new boolean[m][n];
+        StringBuilder sb = new StringBuilder();
+        Set<String> ans = new HashSet<>();
+        for (int i = 0; i < m; i++) {
+            for (int j = 0; j < n; j++) {
+                if (!root.children.containsKey(board[i][j])) continue;
+
+                visited[i][j] = true;
+                sb.append(board[i][j]);
+                dfs(m, n, board, visited, i, j, root.children.get(board[i][j]), sb, ans); //
+                sb.deleteCharAt(0);
+                visited[i][j] = false;
+            }
+        }
+
+        return ans.stream().collect(Collectors.toList());
+    }
+
+    private void dfs(int m, int n, char[][] board, boolean[][] visited, int r, int c, Trie curr, StringBuilder sb, Set<String> ans) {
+        if (curr.ends) ans.add(sb.toString());
+
+        int[] dr = {-1, 0, 1, 0};
+        int[] dc = {0, 1, 0, -1};
+
+        for (int i = 0; i < 4; i++) {
+            int nr = r + dr[i];
+            int nc = c + dc[i];
+            if (nr < 0 || nr >= m || nc < 0 || nc >= n || visited[nr][nc]) continue;
+            if (!curr.children.containsKey(board[nr][nc])) continue;
+
+            visited[nr][nc] = true;
+            sb.append(board[nr][nc]);
+            dfs(m, n, board, visited, nr, nc, curr.children.get(board[nr][nc]), sb, ans);
+            sb.deleteCharAt(sb.length() - 1);
+            visited[nr][nc] = false;
+        }
+    }
+}