[Wan] Week 2

3sum/taewanseoul.ts

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+/**
+ * 15. 3Sum
+ * Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0.
+ * Notice that the solution set must not contain duplicate triplets.
+ *
+ * https://leetcode.com/problems/3sum/description/
+ */
+function threeSum(nums: number[]): number[][] {
+  nums.sort((a, b) => a - b);
+  const triplets: number[][] = [];
+
+  for (let i = 0; i < nums.length - 2; i++) {
+    if (nums[i] > 0 || nums[i] === nums[i - 1]) {
+      continue;
+    }
+
+    let low = i + 1;
+    let high = nums.length - 1;
+
+    while (low < high) {
+      const sum = nums[i] + nums[low] + nums[high];
+      if (sum < 0) {
+        low++;
+      } else if (sum > 0) {
+        high--;
+      } else {
+        triplets.push([nums[i], nums[low], nums[high]]);
+
+        while (low < high && nums[low] === nums[low + 1]) {
+          low++;
+        }
+        while (low < high && nums[high] === nums[high - 1]) {
+          high--;
+        }
+        low++;
+        high--;
+      }
+    }
+  }
+
+  return triplets;
+}
+
+// O(n^2) time
+// O(n) space

climbing-stairs/taewanseoul.ts

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+/**
+ * 70. Climbing Stairs
+ * You are climbing a staircase. It takes n steps to reach the top.
+ * Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?
+ *
+ * https://leetcode.com/problems/climbing-stairs/description/
+ */
+function climbStairs(n: number): number {
+  if (n <= 2) {
+    return n;
+  }
+
+  let prev = 1;
+  let cur = 2;
+
+  for (let i = 3; i < n + 1; i++) {
+    const next = prev + cur;
+    prev = cur;
+    cur = next;
+  }
+
+  return cur;
+}
+
+// O(n) time
+// O(1) space

valid-anagram/taewanseoul.ts

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+/**
+ * 242. Valid Anagram
+ * Given two strings s and t, return true if t is an anagram of s, and false otherwise.
+ *
+ * https://leetcode.com/problems/valid-anagram/description/
+ */
+function isAnagram(s: string, t: string): boolean {
+  if (s.length !== t.length) {
+    return false;
+  }
+
+  const charMap = new Map<string, number>();
+
+  for (const char of s) {
+    const count = charMap.get(char);
+    if (count) {
+      charMap.set(char, count + 1);
+    } else {
+      charMap.set(char, 1);
+    }
+  }
+
+  for (const char of t) {
+    const count = charMap.get(char);
+    if (count) {
+      charMap.set(char, count - 1);
+    } else {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+// O(n) time
+// O(n) space