Week4 Solutions

Author: hoyeongkwak

coin-change/hoyeongkwak.java

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+/*
+Time Complexity : O(n)
+Space Complexity : O(n) 
+*/
+
+class Solution {
+    public int coinChange(int[] coins, int amount) {
+        if (amount == 0) return 0;
+        int[] dp = new int[amount + 1];
+        Arrays.fill(dp, Integer.MAX_VALUE);
+        dp[0] = 0;
+        for (int coin : coins) {
+            for (int i = coin; i <= amount; i++) {
+                if (dp[i - coin] != Integer.MAX_VALUE) {
+                    dp[i] = Math.min(dp[i], dp[i - coin] + 1);
+                }   
+            }
+        }
+        return dp[amount] == Integer.MAX_VALUE ? -1 : dp[amount];
+    }
+}

find-minimum-in-rotated-sorted-array/hoyeongkwak.java

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+/*
+Time Complexity : O(log n)
+Space Complexity : O(1) 
+*/
+class Solution {
+    public int findMin(int[] nums) {
+        int left = 0;
+        int right = nums.length - 1;
+
+        while (left < right) {
+            int mid = left + (right - left) / 2;
+            if (nums[mid] > nums[right]) {
+                left = mid + 1;
+            } else {
+                right = mid;
+            }
+        }
+        return nums[left];
+    }
+}

maximum-depth-of-binary-tree/hoyeongkwak.java

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+/**
+ * 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;
+ *     }
+ * }
+ */
+/*
+Time Complexity : O(n)
+Space Complexity : O(log n) 
+*/
+class Solution {
+    public int maxDepth(TreeNode root) {
+        if (root == null) return 0;
+        int maxLeft = maxDepth(root.left);
+        int maxRight = maxDepth(root.right);
+        return Math.max(maxLeft, maxRight) + 1;
+    }
+}

merge-two-sorted-lists/hoyeongkwak.java

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+/*
+Time Complexity : O(m + n)
+Space Complexity : O(1) 
+*/
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+        ListNode tempNode = new ListNode(0);
+        ListNode current = tempNode;
+
+        while (list1 != null && list2 != null) {
+            if (list1.val <= list2.val) {
+                current.next = list1;
+                list1 = list1.next;
+            } else {
+                current.next = list2;
+                list2 = list2.next;
+            }
+            current = current.next;
+        }
+
+        current.next = (list1 != null) ? list1 : list2;
+        return tempNode.next;
+        // if (list1 == null) {
+        //     return list2;
+        // }
+        // if (list2 == null) {
+        //     return list1;
+        // }
+        // if (list1.val < list2.val) {
+        //     list1.next = mergeTwoLists(list1.next, list2);
+        //     return list1;
+        // } else {
+        //     list2.next = mergeTwoLists(list1, list2.next);
+        //     return list2;
+        // }
+    }
+}

word-search/hoyeongkwak.java

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+/*
+n : 셀 수
+L : 단어 길이
+Time Complexity : O(n * 4^L)
+Space Complexity : O(L) 
+*/
+
+class Solution {
+    public boolean exist(char[][] board, String word) {
+        if (board == null || board.length == 0 || board[0].length == 0) return false;
+        int rows = board.length;
+        int cols = board[0].length;
+        boolean[][] visited = new boolean[rows][cols];
+
+        for (int i = 0; i < rows; i++) {
+            for (int j = 0; j < cols; j++) {
+                if (backtrack(board, word, i, j, 0, visited)) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+
+    private boolean backtrack(char[][] board, String word, int row, int col, int index, boolean[][] visited) {
+        if (index == word.length()) return true;
+        if (row < 0 || row >= board.length || col < 0 || col >= board[0].length || visited[row][col] || board[row][col] != word.charAt(index)) {
+            return false;
+        }
+        visited[row][col] = true;
+
+        int[] dx = {-1, 1, 0, 0};
+        int[] dy = {0, 0, -1, 1};
+
+        for (int i = 0; i < 4; i++) {
+            int newRow = row + dx[i];
+            int newCol = col + dy[i];
+
+            if (backtrack(board, word, newRow, newCol, index + 1, visited)) {
+                visited[row][col] = false;
+                return true;
+            }
+        }
+        visited[row][col] = false;
+        return false;
+    }
+}