[강희찬] WEEK 9 Solution

find-minimum-in-rotated-sorted-array/HC-kang.ts

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+/**
+ * https://leetcode.com/problems/find-minimum-in-rotated-sorted-array
+ * T.C. O(log n)
+ * S.C. O(1)
+ */
+function findMin(nums: number[]): number {
+  let left = 0;
+  let right = nums.length - 1;
+  let mid = (left + right) >> 1;
+
+  while (left < right) {
+    if (nums[mid] > nums[right]) {
+      left = mid + 1;
+    } else {
+      right = mid;
+    }
+    mid = (left + right) >> 1;
+  }
+  return nums[left];
+}

linked-list-cycle/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/linked-list-cycle
+ * T.C. O(n)
+ * S.C. O(n)
+ */
+function hasCycle(head: ListNode | null): boolean {
+    const SET = new Set<ListNode>();
+    while (head) {
+        if (SET.has(head)) return true;
+        SET.add(head);
+        head = head.next
+    }
+    return false;
+};
+
+/**
+ * T.C. O(n)
+ * S.C. O(1)
+ */
+function hasCycle(head: ListNode | null): boolean {
+  let fast = head;
+  let slow = head;
+
+  while (fast && fast.next) {
+      fast = fast.next.next;
+      slow = slow!.next;
+
+      if (fast === slow) {
+          return true;
+      }
+  }
+  return false;
+};

maximum-subarray/HC-kang.ts

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+/**
+ * https://leetcode.com/problems/maximum-subarray
+ * Kadane's Algorithm
+ * T.C. O(n)
+ * S.C. O(1)
+ */
+function maxSubArray(nums: number[]): number {
+  let maxSum = nums[0];
+
+  for (let i = 1; i < nums.length; i++) {
+    nums[i] = Math.max(nums[i], nums[i] + nums[i - 1]);
+    maxSum = Math.max(maxSum, nums[i]);
+  }
+
+  return maxSum;
+}
+
+/**
+ * Divide and Conquer version
+ * T.C. O(n log n)
+ * S.C. O(log n) - call stack
+ */
+function maxSubArray(nums: number[]): number {
+  function maxSubArrayHelper(
+    nums: number[],
+    left: number,
+    right: number
+  ): number {
+    if (left === right) return nums[left];
+
+    const mid = (left + right) >> 1;
+    const leftMax = maxSubArrayHelper(nums, left, mid);
+    const rightMax = maxSubArrayHelper(nums, mid + 1, right);
+    let leftSum = -Infinity;
+    let rightSum = -Infinity;
+    let sum = 0;
+
+    for (let i = mid; i >= left; i--) {
+      sum += nums[i];
+      leftSum = Math.max(leftSum, sum);
+    }
+
+    sum = 0;
+    for (let i = mid + 1; i <= right; i++) {
+      sum += nums[i];
+      rightSum = Math.max(rightSum, sum);
+    }
+
+    return Math.max(leftMax, rightMax, leftSum + rightSum);
+  }
+
+  return maxSubArrayHelper(nums, 0, nums.length - 1);
+}
+
+/**
+ * Iterative version
+ * T.C. O(n log n)
+ * S.C. O(log n) - call stack
+ */
+function maxSubArray(nums: number[]): number {
+  const stack = [[0, nums.length - 1]];
+  let maxSum = nums[0];
+
+  while (stack.length) {
+    const [left, right] = stack.pop()!;
+    if (left === right) {
+      maxSum = Math.max(maxSum, nums[left]);
+      continue;
+    }
+
+    const mid = (left + right) >> 1;
+    stack.push([left, mid], [mid + 1, right]);
+
+    let leftSum = -Infinity;
+    let rightSum = -Infinity;
+    let sum = 0;
+
+    for (let i = mid; i >= left; i--) {
+      sum += nums[i];
+      leftSum = Math.max(leftSum, sum);
+    }
+
+    sum = 0;
+    for (let i = mid + 1; i <= right; i++) {
+      sum += nums[i];
+      rightSum = Math.max(rightSum, sum);
+    }
+
+    maxSum = Math.max(maxSum, leftSum + rightSum);
+  }
+
+  return maxSum;
+}

minimum-window-substring/HC-kang.ts

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+/**
+ * https://leetcode.com/problems/minimum-window-substring
+ * T.C. O(s + t)
+ * S.C. O(t)
+ */
+function minWindow(s: string, t: string): string {
+  let minLow = 0;
+  let minHigh = s.length;
+
+  const counts: Record<string, number> = {};
+  for (const c of t) {
+    counts[c] = (counts[c] || 0) + 1;
+  }
+
+  let included = 0;
+
+  let low = 0;
+  for (let high = 0; high < s.length; high++) {
+    if (counts[s[high]]) {
+      if (counts[s[high]] > 0) {
+        included++;
+      }
+      counts[s[high]]--;
+    }
+
+    while (included === t.length) {
+      if (high - low < minHigh - minLow) {
+        minLow = low;
+        minHigh = high;
+      }
+
+      if (counts[s[low]]) {
+        counts[s[low]]++;
+        if (counts[s[low]] > 0) {
+          included--;
+        }
+      }
+
+      low++;
+    }
+  }
+
+  return minHigh === s.length ? '' : s.substring(minLow, minHigh + 1);
+}

pacific-atlantic-water-flow/HC-kang.ts

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+/**
+ * https://leetcode.com/problems/pacific-atlantic-water-flow
+ * T.C. O((m * n)^2)
+ * S.C. O(m * n)
+ */
+function pacificAtlantic(heights: number[][]): number[][] {
+  const dir = [
+    [0, 1],
+    [0, -1],
+    [1, 0],
+    [-1, 0],
+  ];
+
+  function pacificDfs(row: number, col: number, visited: Set<string>) {
+    const key = `${row},${col}`;
+    if (visited.has(key)) return;
+    visited.add(key);
+
+    if (row === 0 || col === 0) {
+      // left or top
+      return true;
+    }
+
+    for (let [r, c] of dir) {
+      if (row + r < 0 || row + r >= heights.length) continue;
+      if (col + c < 0 || col + c >= heights[0].length) continue;
+      if (heights[row][col] < heights[row + r][col + c]) continue;
+      if (pacificDfs(row + r, col + c, visited)) return true;
+    }
+    return false;
+  }
+
+  function atlanticDfs(row: number, col: number, visited: Set<string>) {
+    const key = `${row},${col}`;
+    if (visited.has(key)) return;
+    visited.add(key);
+
+    if (row === heights.length - 1 || col === heights[0].length - 1) {
+      // right or bottom
+      return true;
+    }
+
+    for (let [r, c] of dir) {
+      if (row + r < 0 || row + r >= heights.length) continue;
+      if (col + c < 0 || col + c >= heights[0].length) continue;
+      if (heights[row][col] < heights[row + r][col + c]) continue;
+      if (atlanticDfs(row + r, col + c, visited)) return true;
+    }
+    return false;
+  }
+
+  const result: number[][] = [];
+  for (let i = 0; i < heights.length; i++) {
+    for (let j = 0; j < heights[0].length; j++) {
+      if (
+        pacificDfs(i, j, new Set<string>()) &&
+        atlanticDfs(i, j, new Set<string>())
+      ) {
+        result.push([i, j]);
+      }
+    }
+  }
+
+  return result;
+}
+
+/**
+ * T.C. O(m * n)
+ * S.C. O(m * n)
+ */
+function pacificAtlantic(heights: number[][]): number[][] {
+  const pacific: Set<string> = new Set();
+  const atlantic: Set<string> = new Set();
+
+  const dir = [
+    [0, 1],
+    [0, -1],
+    [1, 0],
+    [-1, 0],
+  ];
+
+  function dfs(row: number, col: number, visited: Set<string>) {
+    const key = `${row},${col}`;
+    if (visited.has(key)) return;
+    visited.add(key);
+
+    for (let [r, c] of dir) {
+      if (row + r < 0 || row + r >= heights.length) continue;
+      if (col + c < 0 || col + c >= heights[0].length) continue;
+      if (heights[row][col] > heights[row + r][col + c]) continue;
+      dfs(row + r, col + c, visited);
+    }
+  }
+
+  for (let i = 0; i < heights.length; i++) {
+    dfs(i, 0, pacific);
+    dfs(i, heights[0].length - 1, atlantic);
+  }
+
+  for (let i = 0; i < heights[0].length; i++) {
+    dfs(0, i, pacific);
+    dfs(heights.length - 1, i, atlantic);
+  }
+
+  const result: number[][] = [];
+
+  for (const p of pacific) {
+    if (atlantic.has(p)) {
+      const [row, col] = p.split(',').map(Number);
+      result.push([row, col]);
+    }
+  }
+
+  return result;
+}