Given a string containing only digits, restore it by returning all possible valid IP address combinations.
For example:
Given"25525511135"
,
return["255.255.11.135", "255.255.111.35"]
. (Order does not matter)
IP地址被三个点分成四块, 每块int值小于256且string不以0开头(除非刚好"0"). 根据这些规则可以用dfs或者循环写.
class Solution {
/*
private void helper(String s, int pos, int remains, String cur, List<String> res) {
if (remains == 0) {
if (s.length() - pos <= 3) {
String sub = s.substring(pos);
int val = Integer.parseInt(sub);
if (!sub.startsWith("0") && val < 256 || sub.equals("0")) {
res.add(cur + "." + s.substring(pos));
}
}
return;
}
for (int i = pos; i < pos + 3 && i < s.length() - 1; i++) {
String sub = s.substring(pos, i + 1);
int val = Integer.parseInt(sub);
if (!sub.startsWith("0") && val < 256 || sub.equals("0")) {
helper(s, i + 1, remains - 1, cur == "" ? sub : cur + "." + sub, res);
}
}
}*/
private boolean isValid(String sub) {
if (sub.length() > 3 || sub.length() == 0 ||sub.charAt(0) == '0' && sub.length() > 1 || Integer.parseInt(sub) > 255) {
return false;
}
return true;
}
public List<String> restoreIpAddresses(String s) {
List<String> res = new ArrayList<>();
int len = s.length();
for (int i = 0; i < 4 && i < len; i++) {
for (int j = i + 1; j < i + 4 && j < len; j++) {
for (int k = j + 1; k < j + 4 && k < len; k++) {
String s1 = s.substring(0, i), s2 = s.substring(i, j),
s3 = s.substring(j, k), s4 = s.substring(k);
if (isValid(s1) && isValid(s2) && isValid(s3) && isValid(s4)) {
res.add(s1 + "." + s2 + "." + s3 + "." + s4);
}
}
}
}
//helper(s, 0, 3, "", res);
return res;
}
}
时间复杂度O(1).