First Occurrence of Target

When to use

• Duplicates present
• Need leftmost index

Invariant

Answer (if exists) is always in [l, r], and we bias left.

Java

static int firstOccurrence(int[] a, int target) {
    int l = 0, r = a.length - 1, ans = -1;
    while (l <= r) {
        int mid = l + (r - l) / 2;
        if (a[mid] == target) {
            ans = mid;
            r = mid - 1;
        } else if (a[mid] < target) {
            l = mid + 1;
        } else {
            r = mid - 1;
        }
    }
    return ans;
}