试探回溯法就行，找到所有数，选出小于n的最大数就行了，复杂度没提要求应该能过； public class Main { public static void main(String[] args) { int n=23121; int[] array=new int[] {2,4,9}; System.out.println(find(n, array)); } public static int find(int n, int[] array) { int depth=0; List<Integer> list=new ArrayList<>(); int path=0; char[] digits=Integer.toString(n).toCharArray(); dfs(n, depth, digits, array, path, list); int result=0; Collections.sort(list); for(int i=0;i<list.size();i++) { if(list.get(i)>=n) { result=list.get(i-1); break; } } return result; } public static void dfs(int n, int depth, char[] digits, int[] array, int path, List<Integer> list) { if(depth==digits.length) { list.add(path); return; } for (int i = 0; i < array.length; i++) { path = path * 10 + array[i]; dfs(n, depth + 1, digits, array, path, list); path = path / 10; } } }