★[leetCode 93] 复原IP地址

这道题我写了两种方法，一种是三重for循环，因为IP地址最多只有四节，那三重for循环就够用了，不像别的回溯题，有n个for循环，不得不用回溯。

class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> ans = new ArrayList<>();
        if (s.length() < 4 || s.length() > 12) return ans;
        for (int i = 1; i <= s.length() && i <= 3; i++) {
            String s1 = s.substring(0, i);
            if (s1.length() > 1 && s1.charAt(0) == '0') continue;
            if (Integer.parseInt(s1)>255) continue;
            for (int j = i + 1; j <= s.length() && j <= i + 3; j++) {
                String s2 = s.substring(i, j);
                if (Integer.parseInt(s2)>255) continue;
                if (s2.length() > 1 && s2.charAt(0) == '0') continue;
                for (int k = j + 1; k <= s.length() && k <= j + 3; k++) {
                    String s3 = s.substring(j, k);
                    String s4 = s.substring(k);
                    if (s3.length() > 1 && s3.charAt(0) == '0') continue;
                    if (s4.length() > 1 && s4.charAt(0) == '0') continue;
                    if (s4.length() > 3 || s4.length() < 1) continue;
                    if (Integer.parseInt(s3)>255||Integer.parseInt(s4)>255) continue;
                    StringBuilder sb = new StringBuilder();
                    sb.append(s1);
                    sb.append(".");
                    sb.append(s2);
                    sb.append(".");
                    sb.append(s3);
                    sb.append(".");
                    sb.append(s4);
                    ans.add(sb.toString());
                }
            }
        }
        return ans;
    }
}

另外就是经典的回溯模版解法

class Solution {
    List<String> ans = new ArrayList<>();
    StringBuilder sb=new StringBuilder();
    public List<String> restoreIpAddresses(String s) {
        traceback(s,0,0);
        return ans;
    }

    public void traceback(String s,int start,int num){
        if (start==s.length()&&num==4){
            ans.add(sb.substring(0,sb.length()-1).toString());
            return;
        }
        if (num>=4||start==s.length()) return;
        for (int i=start+1;i<=s.length()&&i-start<4;i++){
            String s1 = s.substring(start, i);
            if (!isValid(s1)) break;
            sb.append(s1+".");
            traceback(s,i,num+1);
            sb.delete(sb.length()-s1.length()-1,sb.length());
        }

    }
    public boolean isValid(String s){
        if (s.length()<1||s.length()>3||Integer.parseInt(s)>255) return false;
        if (s.length()>1&&s.charAt(0)=='0') return false;
        return true;
    }
}