[강희찬] WEEK 7 Solution

longest-substring-without-repeating-characters/HC-kang.ts

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+/**
+ * https://leetcode.com/problems/longest-substring-without-repeating-characters
+ * T.C. O(n^2)
+ * S.C. O(n)
+ */
+// function lengthOfLongestSubstring(s: string): number {
+//   let max = 0;
+//   for (let i = 0; i < s.length; i++) {
+//     const SET = new Set();
+//     let count = 0;
+//     for (let j = i; j < s.length; j++) {
+//       if (SET.has(s[j])) break;
+//       SET.add(s[j]);
+//       count += 1;
+//       max = Math.max(max, count);
+//     }
+//   }
+//   return max;
+// }
+
+/**
+ * T.C. O(n)
+ * S.C. O(n)
+ */
+// function lengthOfLongestSubstring(s: string): number {
+//   let left = 0;
+//   let right = 0;
+//   let max = 0;
+//   const SET = new Set();
+
+//   while (s[right]) {
+//     if (SET.has(s[right])) {
+//       SET.delete(s[left]);
+//       left++;
+//     } else {
+//       SET.add(s[right]);
+//       max = Math.max(SET.size, max);
+//       right++;
+//     }
+//   }
+
+//   return max;
+// }
+
+/**
+ * T.C. O(n)
+ * S.C. O(n)
+ */
+function lengthOfLongestSubstring(s: string): number {
+  let left = 0;
+  let right = 0;
+  let max = 0;
+  const MAP = new Map<string, number>();
+
+  while (right < s.length) {
+    if (MAP.has(s[right])) {
+      left = Math.max(MAP.get(s[right])! + 1, left);
+    }
+    MAP.set(s[right], right);
+    max = Math.max(max, right - left + 1);
+    right++;
+  }
+
+  return max;
+}

number-of-islands/HC-kang.ts

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+/**
+ * https://leetcode.com/problems/number-of-islands
+ * T.C. O(m*n)
+ * S.C. O(m*n)
+ */
+// function numIslands(grid: string[][]): number {
+//   let count = 0;
+//   let dir = [[1, 0], [0, 1], [-1, 0], [0, -1]];
+
+//   function removeIsland(r: number, c: number) {
+//     if (r < 0 || r >= grid.length) return;
+//     if (c < 0 || c >= grid[0].length) return;
+//     if (grid[r][c] === '0') return;
+
+//     grid[r][c] = '0';
+//     for (let [dr, dc] of dir) {
+//       removeIsland(r + dr, c + dc);
+//     }
+//     return 1;
+//   }
+
+//   for (let r = 0; r < grid.length; r++) {
+//     for (let c = 0; c < grid[0].length; c++) {
+//       if (grid[r][c] === '0') continue;
+//       count++;
+//       removeIsland(r, c);
+//     }
+//   }
+
+//   return count;
+// }
+
+/**
+ * T.C. O(m*n)
+ * S.C. O(m*n)
+ */
+function numIslands(grid: string[][]): number {
+  let count = 0;
+  let dir = [[1, 0], [0, 1], [-1, 0], [0, -1]];
+
+  function removeIsland(r: number, c: number) {
+    const stack = [[r, c]];
+
+    while (stack.length) {
+      const [r, c] = stack.pop()!;
+      if (r < 0 || r >= grid.length) continue;
+      if (c < 0 || c >= grid[0].length) continue;
+      if (grid[r][c] === '0') continue;
+
+      grid[r][c] = '0';
+      for (let [dr, dc] of dir) {
+        stack.push([r + dr, c + dc]);
+      }
+    }
+  }
+
+  for (let r = 0; r < grid.length; r++) {
+    for (let c = 0; c < grid[0].length; c++) {
+      if (grid[r][c] === '0') continue;
+      count++;
+      removeIsland(r, c);
+    }
+  }
+
+  return count;
+}

reverse-linked-list/HC-kang.ts

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+class ListNode {
+  val: number;
+  next: ListNode | null;
+  constructor(val?: number, next?: ListNode | null) {
+    this.val = val === undefined ? 0 : val;
+    this.next = next === undefined ? null : next;
+  }
+}
+
+/**
+ * https://leetcode.com/problems/reverse-linked-list
+ * T.C. O(n)
+ * S.C. O(1)
+ */
+function reverseList(head: ListNode | null): ListNode | null {
+  let prev: ListNode | null = null;
+  let current: ListNode | null = head;
+  while (current !== null) {
+    const next = current.next;
+    current.next = prev;
+    prev = current;
+    current = next;
+  }
+  return prev;
+}

set-matrix-zeroes/HC-kang.ts

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+/**
+ * https://leetcode.com/problems/set-matrix-zeroes
+ * T.C. O(r * c)
+ * S.C. O(r + c)
+ */
+function setZeroes(matrix: number[][]): void {
+  const r = matrix.length;
+  const c = matrix[0].length;
+
+  const zeroRows = new Set<number>();
+  const zeroCols = new Set<number>();
+
+  for (let i = 0; i < r; i++) {
+    for (let j = 0; j < c; j++) {
+      if (matrix[i][j] === 0) {
+        zeroRows.add(i);
+        zeroCols.add(j);
+      }
+    }
+  }
+
+  for (let i = 0; i < r; i++) {
+    for (let j = 0; j < c; j++) {
+      if (zeroRows.has(i) || zeroCols.has(j)) {
+        matrix[i][j] = 0;
+      }
+    }
+  }
+}
+
+/**
+ * T.C. O(r * c)
+ * S.C. O(1)
+ */
+function setZeroes(matrix: number[][]): void {
+  const r = matrix.length;
+  const c = matrix[0].length;
+
+  let firstRowHasZero = false;
+  let firstColHasZero = false;
+
+  if (matrix[0].some((val) => val === 0)) {
+    firstRowHasZero = true;
+  }
+
+  if (matrix.some((row) => row[0] === 0)) {
+    firstColHasZero = true;
+  }
+
+  for (let i = 1; i < r; i++) {
+    for (let j = 1; j < c; j++) {
+      if (matrix[i][j] === 0) {
+        matrix[i][0] = 0;
+        matrix[0][j] = 0;
+      }
+    }
+  }
+
+  for (let i = 1; i < r; i++) {
+    if (matrix[i][0] === 0) {
+      matrix[i].fill(0);
+    }
+  }
+
+  for (let j = 1; j < c; j++) {
+    if (matrix[0][j] === 0) {
+      matrix.forEach((row) => (row[j] = 0));
+    }
+  }
+
+  if (firstRowHasZero) {
+    matrix[0].fill(0);
+  }
+
+  if (firstColHasZero) {
+    matrix.forEach((row) => (row[0] = 0));
+  }
+}

unique-paths/HC-kang.ts

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+/**
+ * Solution 1. recursive - failed with Stack Overflow
+ */
+function uniquePaths(m: number, n: number): number {
+  function factorialMemo() {
+    const cache = [0, 1];
+    return function factorial(n: number) {
+      if (cache[n]) return cache[n];
+      cache[n] = n * factorial(n - 1);
+      return cache[n];
+    };
+  }
+
+  const factorial = factorialMemo();
+  const total = m + n - 2;
+  const right = m - 1;
+  return Math.round(
+    factorial(total) / (factorial(right) * factorial(total - right))
+  );
+}
+
+/**
+ * Solution 2. for loop (with some 야매.. but it works)
+ * https://leetcode.com/problems/unique-paths
+ * T.C. O(m + n)
+ * S.C. O(m + n)
+ */
+function uniquePaths(m: number, n: number): number {
+  function factorialMemo() {
+    const cache = [1, 1];
+    return function factorial(n: number) {
+      if (cache[n]) return cache[n];
+      let result = cache[cache.length - 1];
+      for (let i = cache.length; i <= n; i++) {
+        result = result * i;
+        cache[i] = result;
+      }
+      return result;
+    };
+  }
+
+  const factorial = factorialMemo();
+  const total = m + n - 2;
+  const right = m - 1;
+  return Math.round(
+    factorial(total) / (factorial(right) * factorial(total - right))
+  );
+}
+
+/**
+ * Solution 3. DP
+ * T.C. O(m * n)
+ * S.C. O(m * n)
+ */
+function uniquePaths(m: number, n: number): number {
+  const dp: number[][] = Array.from({ length: m }, () => Array(n).fill(1));
+
+  for (let i = 1; i < m; i++) {
+    for (let j = 1; j < n; j++) {
+      dp[i][j] = dp[i - 1][j] + dp[i][j - 1];
+    }
+  }
+
+  return dp[m - 1][n - 1];
+}