Codeforces Round #365 (Div. 2) - D. Mishka and Interesting sum

Problem Statement:
http://codeforces.com/contest/703/problem/D

Summary:
Given array a[i] for N <= 10^6 integers, and M <= 10^6 queries (L, R): collect all integers in a[L..R] that occur even number of times, and compute their XOR. E.g. for a = {1, 2, 2, 3, 4, 4}, query (2, 7) returns 2 XOR 4 = 6. If no such element in a particular query, return 0.