LeetCode //C - 796. Rotate String

796. Rotate String

Given two strings s and goal, return true if and only if s can become goal after some number of shifts on s.

A shift on s consists of moving the leftmost character of s to the rightmost position.

• For example, if s = “abcde”, then it will be “bcdea” after one shift.
   

Example 1:

Input: s = “abcde”, goal = “cdeab”
Output: true

Example 2:

Input: s = “abcde”, goal = “abced”
Output: false

Constraints:

• 1 <= s.length, goal.length <= 100
• s and goal consist of lowercase English letters.

From: LeetCode
Link: 796. Rotate String

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Solution:

Ideas:

• If two strings s and goal are of different lengths, return false immediately.

• If we concatenate s with itself (e.g., “abcde” + “abcde” = “abcdeabcde”), then all possible rotations of s will appear as substrings of this concatenated string.

• Therefore, if goal is a substring of s + s, then it is a valid rotation of s.

Code:

bool rotateString(char* s, char* goal) {
    int len_s = strlen(s);
    int len_g = strlen(goal);
    
    if (len_s != len_g) return false;

    // Allocate space for concatenated string
    char double_s[201]; // 2*100 + 1 (for null terminator)
    strcpy(double_s, s);
    strcat(double_s, s);

    return strstr(double_s, goal) != NULL;
}