本題沒有考慮到小於1000的情況，當小於1000的時需要給vector的向量中推入0，直到向量中有四位數字，之後再進行排序並進行相關計算

 1 #include <iostream>
 2 #include <algorithm>
 3 #include <vector>
 4 #include <cmath>
 5 using namespace std;
 6 
 7 bool compare(int a, int b) {
 8     return a > b;
 9 }
10 
11 int main() {
12     vector<int
 
> num;
13     int n = 0;
14     cin >> n;
15     while (1) {
16         int chu = n, yu = 0;
17         while (chu >= 10) {
18             yu = chu % 10;
19             chu /= 10;
20             num.push_back(yu);
21         }
22         num.push_back(chu);
23         if (num.size() < 4) {
24
 
             for (int i = num.size(); i < 4; i++)
25                 num.push_back(0);
26         }
27         sort(num.begin(), num.end(), compare);
28         int large = 0;
29         int jie = num.size() - 1;
30         for (int i = 0; i < num.size(); i++) {
31             large += num[i] * pow(10
 
, jie);
32             jie--;
33         }
34         if (large < 1000) {
35             if (large > 100)
36                 cout << ‘0‘;
37             else if (large > 10)
38                 cout << "00";
39             else
40                 cout << "000";
41         }
42         cout << large << " - ";
43         sort(num.begin(), num.end());
44         int little = 0;
45         jie = num.size() - 1;
46         for (int i = 0; i < num.size(); i++) {
47             little += num[i] * pow(10, jie);
48             jie--;
49         }
50         if (little < 1000) {
51             if (little > 100)
52                 cout << ‘0‘;
53             else if (little > 10)
54                 cout << "00";
55             else
56                 cout << "000";
57         }
58         cout << little << " = ";
59         if (large - little < 1000) {
60             if (large - little > 100)
61                 cout << ‘0‘;
62             else if (large - little > 10)
63                 cout << "00";
64             else
65                 cout << "000";
66         }
67         cout << large - little << endl;
68         if (large - little == 6174)
69             break;
70         else if (large == little)
71             break;
72         else {
73             n = large - little;
74             num.clear();
75         }
76     }
77 
78     return 0;
79 }

PAT 乙級 1019