The one of the greatest common subword.cc

#include <iostream>
#include <vector>

using namespace std;

bool less_than(
	const string& a,
	int ai,
	int bi,
	int size
){
	while (ai < size and bi < size) {
		if (a[ai] != a[bi]) return a[ai] < a[bi];

		ai++;
		bi++;
	}

	return true;
}

int main()
{
	string a, b;
	int max, index;

	//vector<vector<int>> subs(501, vector<int>(501));
	int subs[501][501];

	while (cin >> a >> b) {
		max = index = 0;

		for (int i = 0; i <= a.size(); i++) {
			subs[i][0] = 0;
		}
		for (int j = 0; j <= b.size(); j++) {
			subs[0][j] = 0;
		}

		for (int i = 1; i <= a.size(); i++) {
			for (int j = 1; j <= b.size(); j++) {
				if (a[i - 1] == b[j - 1]) {
					subs[i][j] = subs[i - 1][j - 1] + 1;
					if (subs[i][j] > max) {
						max = subs[i][j];
						index = i - max;
					}
					else if (subs[i][j] == max and less_than(a, i - max, index, a.size())) {
						index = i - max;
					}
				}
				else subs[i][j] = 0;
			}
		}

		for (int i = index; i < index + max; i++) {
			cout << a[i];
		}
		cout << endl;
	}
}