/**
 * 返回 所有的 有效的 IP 地址
 */
class Solution {
    int count = 4;
    List<String> segments = new LinkedList<>();
    List<String> result;    

    public List<String> restoreIpAddresses(String s) {
        result = new ArrayList<String>();
        backtrack(s, 0, 0);
        
 
return result;
    }

    public void backtrack(String s, int startIndex, int pointNum) {
        // 明確分割為四段 故不能用startIndex移動到s的尾部作為回溯停止的標準
        if (pointNum == count) {
            if (startIndex == s.length()) {
                StringBuilder ipAddress = new StringBuilder();
                for (int
 
 i = 0; i < count; i++) {
                    ipAddress.append(segments.get(i));
                    if (i != count - 1) {
                        ipAddress.append(".");
                    }
                }
                result.add(ipAddress.toString());
            }
            return ;
        }

        
 
for (int i = startIndex; i < s.length(); i++) {
            if (!isValid(s, startIndex, i)) continue;   // 判斷[startIndex, i]是否為合法欄位
            segments.add(s.substring(startIndex, i+1));
            pointNum ++;
            backtrack(s, i + 1, pointNum);
            pointNum --;
            segments.remove(segments.size() - 1);
        }
    }

    // 判斷s[startIndex, i]區間的字串是否為合理
    public boolean isValid(String s, int left, int right) {
        if (left > right) return false;
        if (s.charAt(left) == '0' && left != right) return false;   // 除了"0" 其餘不能以"0"開頭
        int num = 0;
        for (int i = left; i <= right; i++) {
            if (s.charAt(i) > '9' || s.charAt(i) < '0') return false;
            num = num * 10 + (int)(s.charAt(i) - '0');
            if (num > 255) return false;
        }
        return true;
    }
}