From 33af494ab692f628337ae95a8b4238a93ab1f4ac Mon Sep 17 00:00:00 2001
From: whewchews <whewchews@gmail.com>
Date: Fri, 23 Aug 2024 23:42:12 +0900
Subject: [PATCH 1/6] 1. valid-anagram
---
valid-anagram/whewchews.ts | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
create mode 100644 valid-anagram/whewchews.ts
diff --git a/valid-anagram/whewchews.ts b/valid-anagram/whewchews.ts
new file mode 100644
index 000000000..72b630d7d
--- /dev/null
+++ b/valid-anagram/whewchews.ts
@@ -0,0 +1,21 @@
+function isAnagram(s: string, t: string): boolean {
+ // TC: O(1)
+ if (s.length !== t.length) return false;
+
+ // SC: O(N)
+ const count: { [key: string]: number } = {};
+
+ // TC: O(N)
+ for (let i = 0; i <= s.length - 1; i++) {
+ const sChar = s[i];
+ const tChar = t[i];
+ count[sChar] = (count[sChar] || 0) + 1;
+ count[tChar] = (count[tChar] || 0) - 1;
+ }
+
+ // TC: O(N)
+ return Object.values(count).every((v) => v === 0);
+}
+
+// TC: O(N)
+// SC: O(N)
From 31b7ee60a913b177662ac5abb59054275069aa6c Mon Sep 17 00:00:00 2001
From: whewchews <whewchews@gmail.com>
Date: Sat, 24 Aug 2024 01:34:08 +0900
Subject: [PATCH 2/6] 2: counting bits
---
counting-bits/whewchews.ts | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
create mode 100644 counting-bits/whewchews.ts
diff --git a/counting-bits/whewchews.ts b/counting-bits/whewchews.ts
new file mode 100644
index 000000000..b243840f7
--- /dev/null
+++ b/counting-bits/whewchews.ts
@@ -0,0 +1,19 @@
+function countBits(n: number): number[] {
+ // SC: O(N)
+ const ans = Array(n + 1).fill(0);
+ // TC: O(N)
+ for (let i = 1; i <= n; i++) {
+ let k = i;
+
+ // TC: O(log N)
+ while (k > 0) {
+ ans[i] += k % 2;
+ k = Math.floor(k / 2);
+ }
+ }
+
+ return ans;
+}
+
+// TC: O(N log N)
+// SC: O(N)
From 8f4e3438f854052e245417d7e83ee5643d9712e8 Mon Sep 17 00:00:00 2001
From: whewchews <whewchews@gmail.com>
Date: Sat, 24 Aug 2024 02:24:20 +0900
Subject: [PATCH 3/6] 4.
construct-binary-tree-from-preorder-and-inorder-traversal
---
.../whewchews.ts | 32 +++++++++++++++++++
1 file changed, 32 insertions(+)
create mode 100644 construct-binary-tree-from-preorder-and-inorder-traversal/whewchews.ts
diff --git a/construct-binary-tree-from-preorder-and-inorder-traversal/whewchews.ts b/construct-binary-tree-from-preorder-and-inorder-traversal/whewchews.ts
new file mode 100644
index 000000000..4d54a44f6
--- /dev/null
+++ b/construct-binary-tree-from-preorder-and-inorder-traversal/whewchews.ts
@@ -0,0 +1,32 @@
+class TreeNode {
+ val: number;
+ left: TreeNode | null;
+ right: TreeNode | null;
+ constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+ this.val = val === undefined ? 0 : val;
+ this.left = left === undefined ? null : left;
+ this.right = right === undefined ? null : right;
+ }
+}
+
+function buildTree(preorder: number[], inorder: number[]): TreeNode | null {
+ // build 함수가 각 노드마다 호출됨(N) * 각 노드마다 shift, indexOf 수행(N) = O(N^2)
+ function build(preorder, inorder) {
+ if (inorder.length) {
+ // TC: O(N)
+ const idx = inorder.indexOf(preorder.shift());
+ const root = new TreeNode(inorder[idx]);
+
+ root.left = build(preorder, inorder.slice(0, idx));
+ root.right = build(preorder, inorder.slice(idx + 1));
+
+ return root;
+ }
+ return null;
+ }
+
+ return build(preorder, inorder);
+}
+
+// TC: O(N^2)
+// SC: O(N^2)
From 9529512e72ae5fa4fef424919e466463b048fbe4 Mon Sep 17 00:00:00 2001
From: whewchews <whewchews@gmail.com>
Date: Sat, 24 Aug 2024 03:07:05 +0900
Subject: [PATCH 4/6] 5. decode-ways
---
decode-ways/whewchews.ts | 33 +++++++++++++++++++++++++++++++++
1 file changed, 33 insertions(+)
create mode 100644 decode-ways/whewchews.ts
diff --git a/decode-ways/whewchews.ts b/decode-ways/whewchews.ts
new file mode 100644
index 000000000..77475c769
--- /dev/null
+++ b/decode-ways/whewchews.ts
@@ -0,0 +1,33 @@
+function numDecodings(s: string): number {
+ // SC: O(N)
+ const memo: { [key: number]: number } = { [s.length]: 1 };
+
+ // TC: O(N)
+ const dfs = (start: number): number => {
+ if (start in memo) {
+ return memo[start];
+ }
+
+ if (s[start] === "0") {
+ // 0으로 시작하는 경우 가능한 경우의 수가 없음
+ memo[start] = 0;
+ } else if (
+ start + 1 < s.length &&
+ parseInt(s.substring(start, start + 2)) < 27
+ ) {
+ // 다음에 오는 글자가 두글자 이상 있고, start start+1 두글자가 1~26 사이의 값인 경우
+ memo[start] = dfs(start + 1) + dfs(start + 2);
+ } else {
+ // 1글자만 남은 경우 or 첫 두글자가 27보다 큰 경우
+ memo[start] = dfs(start + 1);
+ }
+
+ return memo[start];
+ };
+
+ // SC: 재귀호출 O(N)
+ return dfs(0);
+}
+
+// TC: O(N)
+// SC: O(N)
From 4b4859b3e3d78178962049c4d71af6266c195bef Mon Sep 17 00:00:00 2001
From: whewchews <whewchews@gmail.com>
Date: Sat, 24 Aug 2024 03:37:53 +0900
Subject: [PATCH 5/6] =?UTF-8?q?4.=20binary=20tree=20=EC=84=A4=EB=AA=85=20?=
=?UTF-8?q?=EC=B6=94=EA=B0=80?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
---
.../whewchews.ts | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/construct-binary-tree-from-preorder-and-inorder-traversal/whewchews.ts b/construct-binary-tree-from-preorder-and-inorder-traversal/whewchews.ts
index 4d54a44f6..4e87a4269 100644
--- a/construct-binary-tree-from-preorder-and-inorder-traversal/whewchews.ts
+++ b/construct-binary-tree-from-preorder-and-inorder-traversal/whewchews.ts
@@ -9,6 +9,23 @@ class TreeNode {
}
}
+/**
+ * preorder: [root, left, right]
+ * inorder: [left, root, right]
+ * preorder의 첫번째 값은 root이다. (1. 현재 root 찾기)
+ * 모든 node의 val는 unique하기 때문에 이 값을 기준으로 inorder에서 root의 위치를 찾을 수 있다.
+ *
+ * inorder에서 root의 위치를 찾으면, root를 기준으로 왼쪽은 left subtree, 오른쪽은 right subtree이다. (2. left subtree, right subtree 구분)
+ * inorder: [...left, root, ...right]
+ * root값은 이미 찾았기 때문에 shift로 제거한다.
+ *
+ * 남은 preorder에서 첫번째 값은 left subtree의 root이다. (3. left subtree 구성)
+ * preorder에서 하나씩 shift하면서 왼쪽 트리를 먼저 구성한다.
+ * preorder에서 첫번째 값이 왼쪽 subtree의 root이다. (1. 현재 root 찾기)
+ * inorder에서 root의 위치를 찾아서 왼쪽 subtree를 구성한다. (2. left subtree, right subtree 구분) (3. left subtree 구성)
+ * root 기준 왼쪽 subtree 구성이 끝나면 오른쪽 subtree를 구성한다.
+ * 위 과정을 재귀적으로 반복하면, 전체 트리를 구성할 수 있다. (1-3 과정 반복)
+ */
function buildTree(preorder: number[], inorder: number[]): TreeNode | null {
// build 함수가 각 노드마다 호출됨(N) * 각 노드마다 shift, indexOf 수행(N) = O(N^2)
function build(preorder, inorder) {
From 6843903024c2c0736f2cd30374fe373bdefbe01b Mon Sep 17 00:00:00 2001
From: whewchews <whewchews@gmail.com>
Date: Sat, 24 Aug 2024 13:48:48 +0900
Subject: [PATCH 6/6] 1. replace || with ?? for safer nullish coalescing
---
valid-anagram/whewchews.ts | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/valid-anagram/whewchews.ts b/valid-anagram/whewchews.ts
index 72b630d7d..9751a2a25 100644
--- a/valid-anagram/whewchews.ts
+++ b/valid-anagram/whewchews.ts
@@ -9,8 +9,8 @@ function isAnagram(s: string, t: string): boolean {
for (let i = 0; i <= s.length - 1; i++) {
const sChar = s[i];
const tChar = t[i];
- count[sChar] = (count[sChar] || 0) + 1;
- count[tChar] = (count[tChar] || 0) - 1;
+ count[sChar] = (count[sChar] ?? 0) + 1;
+ count[tChar] = (count[tChar] ?? 0) - 1;
}
// TC: O(N)