PAT甲级 1069 The Black Hole of Numbers

原题链接：

https://pintia.cn/problem-sets/994805342720868352/exam/problems/type/7?problemSetProblemId=994805400954585088&page=0

注意点：

1.将数字重新排列时，要考虑到有0的情况。比如说：4，如果不补上缺失的0，将4拆分后重新构成新的数字进入计算会变成0004-0004=0000，进入循环而超时。

代码：

#include <iostream>
#include <vector>
#include <algorithm>
#include <iomanip>
using namespace std;

#define endl '\n'

int N;
vector<int> digits;
int a, b;

void split(int x) {
	digits.clear();
	while (x != 0) {
		digits.emplace_back(x % 10);
		x /= 10;
	}
	while (digits.size() != 4) {
		digits.emplace_back(0);
	}
}

bool cmp(int& x, int& y) {
	return x > y;
}

int nonIncrease() {
	int res = 0;
	sort(digits.begin(), digits.end(), cmp);
	for (int i = 0; i < 4; i++) {
		res = res * 10 + digits[i];
	}
	return res;
}

int nonDecrease() {
	int res = 0;
	sort(digits.begin(), digits.end());
	for (int i = 0; i < 4; i++) {
		res = res * 10 + digits[i];
	}
	return res;
}

signed main() {
	ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);

	cin >> N;
	split(N);
	if (digits[0] == digits[1] && digits[1] == digits[2] && digits[2] == digits[3]) {
		cout << N << " - " << N << " = 0000";
	}
	else {
		int ans = a - b;
		int temp = N;
		while (ans != 6174) {
			split(temp);
			a = nonIncrease();
			b = nonDecrease();
			ans = a - b;
			temp = ans;
			cout << setw(4) << setfill('0') << a;
			cout << " - ";
			cout << setw(4) << setfill('0') << b;
			cout << " = ";
			cout << setw(4) << setfill('0') << ans << endl;
		}
	}

	return 0;
}