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📜 WAYPOINT LESSON

Checkpoint — Algorithm toolkit

⭐ beginner⏳ 20 min read📍 Lesson 81 of 85

Three classic patterns in one solution: sum-formula gaps, single-use pair sums, and run-length encoding.

Checkpoint: the algorithm toolkit

Task: implement in Solution:

  1. static int MissingNumber(int[] nums) — the array holds n distinct numbers from 0..n with exactly one missing; return it. ([3,0,1] → 2.)
  2. static bool HasPairSum(int[] nums, int target) — true when two distinct elements sum to target (an element may not pair with itself).
  3. static string Compress(string s) — run-length encoding: "aaabbc" → "a3b2c1"; every run's count is written, even 1 ("a" → "a1"); empty → empty.