Day 41 of DSA Problem Solving

Q.no 1 :- Reverse Vowels of a String

Given a string s, reverse only all the vowels in the string and return it.

Example:

Input: s = "IceCreAm"

Output: "AceCreIm"

Approach

1. Two-Pointer Technique
   
   We use two pointers — one starting from the beginning (start) and one from the end (end) — to scan the string from both sides.

2. Create a Vowel Lookup Object
   
   Create an object named vowels containing all vowels in both uppercase and lowercase (i.e., a, e, i, o, u, A, E, I, O, U) to allow quick lookup.

3. Initialize Pointers

   • start: initially points to the first character.
   • end: initially points to the last character.

4. Loop Through the String

   • Use the loop to evaluate the characters at start and end.
   • Use:

    let isStartVowel = vowels[chars[start]];
    let isEndVowel = vowels[chars[end]];
   

• If both are vowels, swap them and move both pointers (start++, end--).
• If isStartVowel is false, increment start to skip non-vowels.
• If isEndVowel is false, decrement end to skip non-vowels.

1. Continue until the start pointer meets or crosses the end pointer.

---

Dry Run

📈 Time and Space Complexity

• Time Complexity: O(n) — each character is checked at most once.
• Space Complexity: O(n) — due to splitting the string into a character array.

Code

/**
 * @param {string} s
 * @return {string}
 */

var reverseVowels = function(s) {
   let vowels = {
        a: true, e: true, i: true, o: true, u: true,
        A: true, E: true, I: true, O: true, U: true
    };


    let chars = s.split('');
    let start = 0;
    let end = s.length - 1;

    while (start < end) {
        let isStartVowel = vowels[chars[start]];
        let isEndVowel = vowels[chars[end]];


        if(isStartVowel && isEndVowel){
           let temp = chars[start]
           chars[start] = chars[end];
           chars[end] = temp;
           start++
           end--
        }else if(!isStartVowel){
           start++
        }else if(!isEndVowel){
            end--
        }


    }

    return chars.join('');

};

Q.no 2 :- Add Digits

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Example

Input: num = 38
Step 1: 3 + 8 = 11
Step 2: 1 + 1 = 2
Output: 2

Approach

There is a math shortcut that helps us jump directly to the final answer without doing all those steps.

Here's the rule:

• If the number is 0, just return 0.

• Otherwise, use this formula:
  

Answer = 1 + ((number - 1) % 9)

⏱️ Time and Space:

• Time: O(1) → Always takes just one step.

• Space: O(1) → Doesn’t use any extra memory.
  

/**
 * @param {string} s
 * @return {string}
 */

var reverseVowels = function(s) {
   let vowels = {
        a: true, e: true, i: true, o: true, u: true,
        A: true, E: true, I: true, O: true, U: true
    };


    let chars = s.split('');
    let start = 0;
    let end = s.length - 1;

    while (start < end) {
        let isStartVowel = vowels[chars[start]];
        let isEndVowel = vowels[chars[end]];


        if(isStartVowel && isEndVowel){
           let temp = chars[start]
           chars[start] = chars[end];
           chars[end] = temp;
           start++
           end--
        }else if(!isStartVowel){
           start++
        }else if(!isEndVowel){
            end--
        }


    }

    return chars.join('');

};