LeetCode //C - 93. Restore IP Addresses

93. Restore IP Addresses

A valid IP address consists of exactly four integers separated by single dots. Each integer is between 0 and 255 (inclusive) and cannot have leading zeros.

• For example, “0.1.2.201” and “192.168.1.1” are valid IP addresses, but “0.011.255.245”, “192.168.1.312” and “192.168@1.1” are invalid IP addresses.

Given a string s containing only digits, return all possible valid IP addresses that can be formed by inserting dots into s. You are not allowed to reorder or remove any digits in s. You may return the valid IP addresses in any order.
 

Example 1:

Input: s = “25525511135”
Output: [“255.255.11.135”,“255.255.111.35”]

Example 2:

Input: s = “0000”
Output: [“0.0.0.0”]

Example 3:

Input: s = “101023”
Output: [“1.0.10.23”,“1.0.102.3”,“10.1.0.23”,“10.10.2.3”,“101.0.2.3”]

Constraints:

• 1 <= s.length <= 20
• s consists of digits only.

From: LeetCode
Link: 93. Restore IP Addresses

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Solution:

Ideas:

• isValid Function: Checks if the substring s[start:end] is a valid segment of an IP address.
• backtrack Function: Recursively tries to build the IP address. It places a dot after 1 to 3 valid digits and moves on to the next segment.
• restoreIpAddresses Function: Initializes necessary data structures and starts the backtracking process.

Code:

/*** Note: The returned array must be malloced, assume caller calls free().*/int isValid(char *s, int start, int end) {if (start > end) {return 0;}// "0" is valid, but "0x", "09", etc. are not validif (s[start] == '0' && start != end) {return 0;}int num = 0;for (int i = start; i <= end; i++) {if (s[i] < '0' || s[i] > '9') { // non-digit charactersreturn 0;}num = num * 10 + (s[i] - '0');if (num > 255) {return 0;}}return 1;
}void backtrack(char *s, int start, int part, char *ip, int len, char **result, int *returnSize) {if (part == 4 && start == strlen(s)) {ip[len - 1] = '\0'; // remove the last dotresult[*returnSize] = strdup(ip);(*returnSize)++;return;}if (part == 4 || start == strlen(s)) {return;}int addressLen = strlen(s);for (int i = 1; i <= 3; i++) {if (start + i > addressLen) {break;}if (isValid(s, start, start + i - 1)) {ip[len] = s[start];if (i > 1) {ip[len + 1] = s[start + 1];}if (i > 2) {ip[len + 2] = s[start + 2];}ip[len + i] = '.';backtrack(s, start + i, part + 1, ip, len + i + 1, result, returnSize);}}
}char** restoreIpAddresses(char* s, int* returnSize) {*returnSize = 0;int len = strlen(s);if (len < 4 || len > 12) {return NULL;}char **result = (char **)malloc(100 * sizeof(char *));char *ip = (char *)malloc((len + 4) * sizeof(char));backtrack(s, 0, 0, ip, 0, result, returnSize);free(ip);return result;
}