354.: Missax

(Typical “find the missing element” problem – often appears on many online judges under the name Missax .) 1. Problem statement You are given an integer N ( 1 ≤ N ≤ 10⁶ ) . Then N distinct integers a₁ , a₂ , … , a_N are supplied.

{ 1 , 2 , 3 , … , N+1 } i.e. the list is a permutation of the numbers 1 … N+1 . Your task is to output the missing number.

x = 1 xor 2 xor … xor (N+1) xor a1 xor a2 … xor aN Every value that appears twice cancels out, leaving the missing number. Both approaches are linear in time and constant in memory. For each test case 354. Missax

N a1 a2 … aN (may be split over several lines) The file ends with a line containing 0 , which must be processed.

Proof. The algorithm first stores missing = S . During the input loop it subtracts each read number a_j from missing . After the loop finishes (Typical “find the missing element” problem – often

read N if N == 0 → finish missing = (N+1)*(N+2)/2 // 64‑bit integer repeat N times read x missing -= x output missing or (XOR version)

Proof. By Lemma 2 the value stored in missing after processing the whole test case equals S – T . By Lemma 1 S – T equals the missing element m . Therefore the printed value is exactly m . ∎ Time – each number is read and processed once → O(N) per test case. Memory – only a few 64‑bit variables are kept → O(1) . 6. Reference implementation (C++17) #include <bits/stdc++.h> using namespace std; { 1 , 2 , 3 , … , N+1 } i

All the numbers belong to the set

The input may contain several test cases. Each test case is described as follows

missing = 0 for i = 1 … N+1 missing ^= i repeat N times read x missing ^= x output missing We prove the sum‑based algorithm; the XOR version follows the same line of reasoning. Lemma 1 Let S = Σ_{i=1}^{N+1} i . Let T = Σ_{j=1}^{N} a_j be the sum of the numbers actually present. If exactly one element m of {1,…,N+1} is missing, then S - T = m .

missing = S – Σ a_j = S – T ∎ For each test case the algorithm outputs the unique missing integer.