[rivkode] WEEK 15 solutions

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

@@ -0,0 +1,51 @@
+/**
+ * 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;
+ *     }
+ * }
+ */
+
+import java.util.*;
+
+class Solution {
+    private int preIdx = 0;
+    private Map<Integer, Integer> inPos = new HashMap<>();
+
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+        // inorder 값의 위치를 미리 저장: value -> index
+        for (int i = 0; i < inorder.length; i++) {
+            inPos.put(inorder[i], i);
+        }
+
+        return build(preorder, 0, inorder.length - 1);
+    }
+
+    private TreeNode build(int[] preorder, int inLeft, int inRight) {
+        // inorder 구간이 비면 서브트리 없음
+        if (inLeft > inRight) return null;
+
+        // preorder에서 현재 루트 꺼내기
+        int rootVal = preorder[preIdx++];
+        TreeNode root = new TreeNode(rootVal);
+
+        // inorder에서 루트 위치 찾기
+        int k = inPos.get(rootVal);
+
+        // 왼쪽 서브트리: inorder[inLeft .. k-1]
+        root.left = build(preorder, inLeft, k - 1);
+
+        // 오른쪽 서브트리: inorder[k+1 .. inRight]
+        root.right = build(preorder, k + 1, inRight);
+
+        return root;
+    }
+}

counting-bits/rivkode.java

@@ -27,26 +27,26 @@ public int[] countBits(int n) {
 입력받은 n에 대해 toBinaryString() 을 사용하여 binaryString 값을 알아낸뒤 charAt()으로 각 인덱스별 접근하여 1의 개수를 찾아내는 방식이다.
  */
 
-class Solution {
-    public int[] countBits(int n) {
-        int[] answer = new int[n + 1];
-        for (int i=0; i<n+1; i++) {
-            String bit = Integer.toBinaryString(i);
-            System.out.println(bit);
-            int cnt = 0;
-            for (int j=0; j < bit.length(); j++) {
-                char c = bit.charAt(j);
-                int num = c-'0';
-                if (num == 1) {
-                    cnt += 1;
-                }
-            }
-
-            answer[i] = cnt;
-        }
-
-        return answer;
-    }
-}
+// class Solution {
+//     public int[] countBits(int n) {
+//         int[] answer = new int[n + 1];
+//         for (int i=0; i<n+1; i++) {
+//             String bit = Integer.toBinaryString(i);
+//             System.out.println(bit);
+//             int cnt = 0;
+//             for (int j=0; j < bit.length(); j++) {
+//                 char c = bit.charAt(j);
+//                 int num = c-'0';
+//                 if (num == 1) {
+//                     cnt += 1;
+//                 }
+//             }
+
+//             answer[i] = cnt;
+//         }
+
+//         return answer;
+//     }
+// }