Rotate String [Easy] — String Contains Trick

Time: O(n²) naïve; O(n) with KMP
Space: O(n) for concatenation

Problem Statement

Given two strings s and goal, return true if s can become goal after some rotation (shifting left by k positions).

Input: s="abcde", goal="cdeab" → true
Input: s="abcde", goal="abced" → false

Any rotation of s is a substring of s + s. Check len(s) == len(goal) and goal in (s+s).

bool rotateString(string s, string goal) {
    return s.size()==goal.size() && (s+s).find(goal)!=string::npos;
}

public boolean rotateString(String s, String goal) {
    return s.length()==goal.length() && (s+s).contains(goal);
}

var rotateString = function(s, goal) {
    return s.length===goal.length && (s+s).includes(goal);
};

def rotateString(s: str, goal: str) -> bool:
    return len(s) == len(goal) and goal in s + s

Use KMP or str.find (Boyer-Moore Horspool internally in most implementations) for true O(n).