Please write pseudocode and code in C++

Write an algorithm that returns the index of the first occurrence of the value key in the sequence, sn If the key is not in the sequence, the algorithm returns the value 0. Example: If the sequence is 12 11 12 23 and the key is 12, the algorithm returns the value 1. (You do not have to compile, you can use C/Java notation if you like instead of pseudo code). Estimate the complexity of your algorithm