JavaScript Program to Check if a string can be obtained by rotating another string d places

Rotating a string means moving characters to the left or right by a specified number of positions. We'll check if one string can be obtained by rotating another string exactly d places in either direction.

Problem Understanding

Given two strings and a rotation count d, we need to determine if the first string can be transformed into the second string by rotating it d positions left or right.

Example

String 1: "abcdef", String 2: "defabc", Rotations: 3

Left rotation by 3: "abcdef" ? "bcdefa" ? "cdefab" ? "defabc" ?

Approach 1: Left Rotation

In left rotation, we move characters from the beginning to the end. We use the substring method to split and rearrange the string.

// Function to perform left rotation
function leftRotation(str, k) {
   var len = str.length;
   k = k % len; // Handle rotations greater than string length
   return str.substring(k) + str.substring(0, k);
}

// Function to check if string can be converted by left rotation
function checkLeftRotation(string1, string2, rotations) {
   var len1 = string1.length;
   var len2 = string2.length;
   
   // Strings must have equal length
   if (len1 !== len2) {
      return false;
   }
   
   // Rotate first string and compare
   var rotatedString = leftRotation(string1, rotations);
   return rotatedString === string2;
}

// Test the function
var string1 = "abcdef";
var string2 = "defabc";
var rotations = 3;

if (checkLeftRotation(string1, string2, rotations)) {
   console.log("Yes, '" + string1 + "' can be converted to '" + string2 + "' by " + rotations + " left rotations");
} else {
   console.log("No, conversion not possible with " + rotations + " left rotations");
}

Yes, 'abcdef' can be converted to 'defabc' by 3 left rotations

Approach 2: Right Rotation

In right rotation, we move characters from the end to the beginning. The logic is reversed compared to left rotation.

// Function to perform right rotation
function rightRotation(str, k) {
   var len = str.length;
   k = k % len; // Handle rotations greater than string length
   return str.substring(len - k) + str.substring(0, len - k);
}

// Function to check if string can be converted by right rotation
function checkRightRotation(string1, string2, rotations) {
   var len1 = string1.length;
   var len2 = string2.length;
   
   // Strings must have equal length
   if (len1 !== len2) {
      return false;
   }
   
   // Rotate first string and compare
   var rotatedString = rightRotation(string1, rotations);
   return rotatedString === string2;
}

// Test the function
var string1 = "abcdef";
var string2 = "bcdefa";
var rotations = 1;

if (checkRightRotation(string1, string2, rotations)) {
   console.log("Yes, '" + string1 + "' can be converted to '" + string2 + "' by " + rotations + " right rotation(s)");
} else {
   console.log("No, conversion not possible with " + rotations + " right rotation(s)");
}

Yes, 'abcdef' can be converted to 'bcdefa' by 1 right rotation(s)

Combined Approach: Check Both Directions

Often we need to check if rotation is possible in either direction without specifying which one.

function canRotateString(str1, str2, d) {
   // Check if lengths are equal
   if (str1.length !== str2.length) {
      return { possible: false, direction: "none" };
   }
   
   // Check left rotation
   var leftRotated = leftRotation(str1, d);
   if (leftRotated === str2) {
      return { possible: true, direction: "left" };
   }
   
   // Check right rotation
   var rightRotated = rightRotation(str1, d);
   if (rightRotated === str2) {
      return { possible: true, direction: "right" };
   }
   
   return { possible: false, direction: "none" };
}

// Test cases
var testCases = [
   { str1: "abcdef", str2: "defabc", rotations: 3 },
   { str1: "hello", str2: "llohe", rotations: 2 },
   { str1: "abc", str2: "def", rotations: 1 }
];

testCases.forEach(function(test, index) {
   var result = canRotateString(test.str1, test.str2, test.rotations);
   console.log("Test " + (index + 1) + ": " + 
               (result.possible ? 
                "Possible via " + result.direction + " rotation" : 
                "Not possible"));
});

Test 1: Possible via left rotation
Test 2: Possible via left rotation
Test 3: Not possible

Time and Space Complexity

Key Points

• Use modulo operation (k % len) to handle rotations greater than string length
• Always check if strings have equal length before rotation
• Left rotation: move first k characters to end
• Right rotation: move last k characters to beginning

Conclusion

String rotation problems can be solved efficiently using substring operations. The key is understanding the direction of rotation and using modulo arithmetic to handle edge cases. Both approaches have linear time complexity and are practical for most use cases.