Binary Search First Occurrence

Find the first copy of a duplicated target by recording matches and continuing to search the left half.

Algorithm

Basic Implementation

basic.cs

using System;

class Program {
	static void Main() {
		int[] arr = new int[] { 1, 2, 4, 4, 4, 7, 9 };
		int target = 4;
		int lo = 0;
		int hi = arr.Length - 1;
		int result = -1;
		while (lo <= hi) {
			int mid = lo + (hi - lo) / 2;
			if (arr[mid] == target) {
				result = mid;
				hi = mid - 1;
			} else if (arr[mid] < target) {
				lo = mid + 1;
			} else {
				hi = mid - 1;
			}
		}
		Console.WriteLine(result);
	}
}

Complexity

Time: O(log n)
Space: O(1)

Implementation notes

Keep the explicit loop instead of calling BCL search helpers so the replay can show lo, hi, mid, and the saved result after each comparison.
int mid = lo + (hi - lo) / 2 avoids overflowing the midpoint addition, and each arr[mid] read from the managed reference int[] is bounds-checked by the CLR.
On equality, C# records result = mid and moves hi = mid - 1, preserving the duplicate-left search rather than stopping at any matching index.

execution replay The checked-in replay follows the language-neutral state table for `search-binary-first`.

cross-language comparison This C# DSA version keeps the same data and final output as every other DSA book in this wave.