[bky373] 8th Week Solutions

combination-sum/bky373.java

@@ -0,0 +1,28 @@
+/*
+ * time: O(n^2)
+ * space: O(n)
+ */
+class Solution {
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> result = new ArrayList<>();
+        List<Integer> combination = new LinkedList<>();
+        backtrack(candidates, target, combination, 0, result);
+        return result;
+    }
+
+    void backtrack(int[] candidates, int remain, List<Integer> combination, int startIndex, List<List<Integer>> result) {
+        if (remain == 0) {
+            result.add(new ArrayList<>(combination));
+            return;
+        } else if (remain < 0) {
+            return;
+        }
+
+        for (int i = startIndex; i < candidates.length; i++) {
+            combination.add(candidates[i]);
+            backtrack(candidates, remain - candidates[i], combination, i, result);
+            combination.removeLast();
+        }
+    }
+}

construct-binary-tree-from-preorder-and-inorder-traversal/bky373.java

@@ -0,0 +1,31 @@
+/*
+ * time: O(N)
+ * space: O(N)
+ */
+class Solution {
+
+    int preorderIndex;
+    Map<Integer, Integer> inorderIndexMap;
+
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+        preorderIndex = 0;
+        inorderIndexMap = new HashMap<>();
+        for (int i = 0; i < inorder.length; i++) {
+            inorderIndexMap.put(inorder[i], i);
+        }
+        return arrayToTree(preorder, 0, preorder.length - 1);
+    }
+
+    private TreeNode arrayToTree(int[] preorder, int left, int right) {
+        if (left > right) {
+            return null;
+        }
+
+        int rootValue = preorder[preorderIndex++];
+        TreeNode root = new TreeNode(rootValue);
+
+        root.left = arrayToTree(preorder, left, inorderIndexMap.get(rootValue) - 1);
+        root.right = arrayToTree(preorder, inorderIndexMap.get(rootValue) + 1, right);
+        return root;
+    }
+}

implement-trie-prefix-tree/bky373.java

@@ -0,0 +1,94 @@
+/*
+ * time: O(M)
+ * space: O(N*M)
+ */
+public class TrieNode {
+    private static final int MAX_SIZE = 26;
+
+    public TrieNode[] subNodes;
+    public boolean isEndOfWord;
+
+    public TrieNode() {
+        subNodes = new TrieNode[MAX_SIZE];
+    }
+
+    public boolean contains(char ch) {
+        return subNodes[ch - 'a'] != null;
+    }
+
+    public TrieNode getSubNode(char ch) {
+        return subNodes[ch - 'a'];
+    }
+
+    public void put(char ch, TrieNode node) {
+        subNodes[ch - 'a'] = node;
+    }
+
+    @Override
+    public String toString() {
+        return "TrieNode{sub=" + Arrays.toString(subNodes) + "}";
+    }
+}
+
+
+class Trie {
+
+    public TrieNode root;
+
+    public Trie() {
+        root = new TrieNode();
+    }
+
+    public void insert(String word) {
+        TrieNode node = root;
+        for (int i = 0; i < word.length(); i++) {
+            char ch = word.charAt(i);
+            if (!node.contains(ch)) {
+                node.put(ch, new TrieNode());
+            }
+            node = node.getSubNode(ch);
+        }
+        node.isEndOfWord = true;
+    }
+
+    public boolean search(String word) {
+        TrieNode node = root;
+        for (int i = 0; i < word.length(); i++) {
+            if (node == null) {
+                return false;
+            }
+            if (!node.contains(word.charAt(i))) {
+                return false;
+            }
+            node = node.getSubNode(word.charAt(i));
+            if (i == word.length() - 1 && node.isEndOfWord) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public boolean startsWith(String prefix) {
+        TrieNode node = root;
+        for (int i = 0; i < prefix.length(); i++) {
+            if (node == null) {
+                return false;
+            }
+            if (!node.contains(prefix.charAt(i))) {
+                return false;
+            }
+            node = node.getSubNode(prefix.charAt(i));
+        }
+        return true;
+    }
+
+
+}
+
+/**
+ * Your Trie object will be instantiated and called as such:
+ * Trie obj = new Trie();
+ * obj.insert(word);
+ * boolean param_2 = obj.search(word);
+ * boolean param_3 = obj.startsWith(prefix);
+ */

kth-smallest-element-in-a-bst/bky373.java

@@ -0,0 +1,22 @@
+/*
+ * time: O(N)
+ * space: O(N)
+ */
+class Solution {
+
+    List<Integer> inorderList = new ArrayList<>();
+
+    public int kthSmallest(TreeNode root, int k) {
+        inorder(root);
+        return inorderList.get(k - 1);
+    }
+
+    public void inorder(TreeNode cur) {
+        if (cur == null) {
+            return;
+        }
+        inorder(cur.left);
+        inorderList.add(cur.val);
+        inorder(cur.right);
+    }
+}

word-search/bky373.java

@@ -0,0 +1,52 @@
+/*
+ * time: O(N * 4^L)
+ *  - N: the number of cells in the board
+ *  - L: the length of the word
+ *  - 4: the length of directions
+ * space: O(L)
+ *  - L: the recursive backtracking function call stacks.
+ */
+class Solution {
+
+    private char[][] board;
+    private int rLen;
+    private int cLen;
+
+    public boolean exist(char[][] board, String word) {
+        this.board = board;
+        this.rLen = board.length;
+        this.cLen = board[0].length;
+
+        for (int row = 0; row < this.rLen; ++row) {
+            for (int col = 0; col < this.cLen; ++col) {
+                if (this.backtrack(row, col, word, 0)) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+
+    private boolean backtrack(int row, int col, String word, int matchIndex) {
+        if (matchIndex >= word.length()) {
+            return true;
+        }
+
+        if (row < 0 || row == this.rLen || col < 0 || col == this.cLen || this.board[row][col] != word.charAt(matchIndex)) {
+            return false;
+        }
+
+        this.board[row][col] = ' ';
+
+        int[] rowDirs = {0, 1, 0, -1};
+        int[] colDirs = {1, 0, -1, 0};
+        for (int dir = 0; dir < 4; ++dir) {
+            if (backtrack(row + rowDirs[dir], col + colDirs[dir], word, matchIndex + 1)) {
+                return true;
+            }
+        }
+
+        this.board[row][col] = word.charAt(matchIndex);
+        return false;
+    }
+}