|
|
package class36;
|
|
|
|
|
|
import java.util.TreeSet;
|
|
|
|
|
|
// 来自腾讯
|
|
|
// 给定一个字符串str，和一个正数k
|
|
|
// 返回长度为k的所有子序列中，字典序最大的子序列
|
|
|
public class Code09_MaxKLenSequence {
|
|
|
|
|
|
public static String maxString(String s, int k) {
|
|
|
if (k <= 0 || s.length() < k) {
|
|
|
return "";
|
|
|
}
|
|
|
char[] str = s.toCharArray();
|
|
|
int n = str.length;
|
|
|
char[] stack = new char[n];
|
|
|
int size = 0;
|
|
|
for (int i = 0; i < n; i++) {
|
|
|
while (size > 0 && stack[size - 1] < str[i] && size + n - i > k) {
|
|
|
size--;
|
|
|
}
|
|
|
if (size + n - i == k) {
|
|
|
return String.valueOf(stack, 0, size) + s.substring(i);
|
|
|
}
|
|
|
stack[size++] = str[i];
|
|
|
}
|
|
|
return String.valueOf(stack, 0, k);
|
|
|
}
|
|
|
|
|
|
// 为了测试
|
|
|
public static String test(String str, int k) {
|
|
|
if (k <= 0 || str.length() < k) {
|
|
|
return "";
|
|
|
}
|
|
|
TreeSet<String> ans = new TreeSet<>();
|
|
|
process(0, 0, str.toCharArray(), new char[k], ans);
|
|
|
return ans.last();
|
|
|
}
|
|
|
|
|
|
// 为了测试
|
|
|
public static void process(int si, int pi, char[] str, char[] path, TreeSet<String> ans) {
|
|
|
if (si == str.length) {
|
|
|
if (pi == path.length) {
|
|
|
ans.add(String.valueOf(path));
|
|
|
}
|
|
|
} else {
|
|
|
process(si + 1, pi, str, path, ans);
|
|
|
if (pi < path.length) {
|
|
|
path[pi] = str[si];
|
|
|
process(si + 1, pi + 1, str, path, ans);
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
|
|
|
// 为了测试
|
|
|
public static String randomString(int len, int range) {
|
|
|
char[] str = new char[len];
|
|
|
for (int i = 0; i < len; i++) {
|
|
|
str[i] = (char) ((int) (Math.random() * range) + 'a');
|
|
|
}
|
|
|
return String.valueOf(str);
|
|
|
}
|
|
|
|
|
|
public static void main(String[] args) {
|
|
|
int n = 12;
|
|
|
int r = 5;
|
|
|
int testTime = 10000;
|
|
|
System.out.println("测试开始");
|
|
|
for (int i = 0; i < testTime; i++) {
|
|
|
int len = (int) (Math.random() * (n + 1));
|
|
|
String str = randomString(len, r);
|
|
|
int k = (int) (Math.random() * (str.length() + 1));
|
|
|
String ans1 = maxString(str, k);
|
|
|
String ans2 = test(str, k);
|
|
|
if (!ans1.equals(ans2)) {
|
|
|
System.out.println("出错了！");
|
|
|
System.out.println(str);
|
|
|
System.out.println(k);
|
|
|
System.out.println(ans1);
|
|
|
System.out.println(ans2);
|
|
|
break;
|
|
|
}
|
|
|
}
|
|
|
System.out.println("测试结束");
|
|
|
}
|
|
|
|
|
|
}
|
