For each of 0 , 1 , \(\ldots\) , and 9, maintain a flag that signifies whether the character appears in the string \(S\), and finally output the missing one. There are various ways to convert a character or a string to a number depending on programming languages (or even in a single language); it may be useful to remember your favorite way to do so.

Sample code in C++:

#include<bits/stdc++.h>
using namespace std;

int main(void) {
	string S;
	bool flag[10];
	for (int i = 0; i < 10; i++)flag[i] = true;
	cin >> S;
	for (int i = 0; i < 9; i++)flag[((int)(S[i] - '0'))] = false;
	for (int i = 0; i < 10; i++)if (flag[i])cout << i << endl;
	return 0;
}

Sample code in Python:

S=input()
flag=[True for i in range(10)]
for i in range(9):
	flag[int(S[i])]=False
for i in range(10) :
	if(flag[i]):
		print(i)

By the way, the answer can be found by subtracting all the given digits from \(0+1+\cdots +9=45\).

Sample code in C++:

#include<bits/stdc++.h>
using namespace std;

int main(void) {
	string S;
	cin >> S;
	int ans = 45;
	for (int i = 0; i < 9; i++)ans -= ((int)(S[i] - '0'));
	cout << ans << endl;
	return 0;
}