First Occurrence

Given a target integer T and an integer array A sorted in ascending order, find the index of the first occurrence of T in A or return -1 if there is no such index.

Assumptions

• There can be duplicate elements in the array.

Examples

• A = {1, 2, 3}, T = 2, return 1
• A = {1, 2, 3}, T = 4, return -1
• A = {1, 2, 2, 2, 3}, T = 2, return 1

Corner Cases

• What if A is null or A of zero length? We should return -1 in this case.

time: O(log(n)), space: O(1)

public class Solution {
  public int firstOccur(int[] array, int target) {
    // Write your solution here
    if(array == null || array.length == 0) return -1;
    int left = 0, right = array.length - 1;
    while(left + 1 < right) {
      int mid = left + (right - left) / 2;
      if(array[mid] == target)
        right = mid;
      else if(array[mid] < target)
        left = mid;
      else
        right = mid;
    }
    if(array[left] == target)
      return left;
    else if(array[right] == target)
      return right;
    else
      return -1;
  }
}