Finding missing number

Given a list of sequence number in an array, starting from 1 - N, find the missing number.

Solution#1:
If array is sorted, O(log(n)) will be the best approach. Divide the number to half, compare the value of middle element with the index+1. If the same, missing number is located in left, else it is located in right. Then divide the half to another half and proceed with same approach as above.

Solution#2:
If array is unsorted, N = length of array+1, sum = N*(N+1)/2. Then loop through the array and minus the value of array with sum. At the end of loop, the remaining value in sum is the missing number. O(N) approach.