Find Missing Number

1. Given a set of positive numbers less than equal to N, where one number is missing. Find the missing number efficiently.
2. Given a set of positive numbers less than equal to N, where two numbers are missing. Find the missing numbers efficiently.
3. Given a sequence of positive numbers less than equal to N, where one number is repeated and another is missing. Find the repeated and the missing numbers efficiently.
4. Given a sequence of integers (positive and negative). Find the first missing positive number in the sequence.