Arrays & Strings

Arrays in Competitive Coding

Arrays are the most fundamental data structure. Understanding array manipulation is crucial for solving many problems efficiently.

Common Array Operations

// Traversal
for (int i = 0; i < arr.length; i++) {
    // Process arr[i]
}

// Enhanced for loop
for (int num : arr) {
    // Process num
}

// Two pointers technique
int left = 0, right = arr.length - 1;
while (left < right) {
    // Process arr[left] and arr[right]
    left++;
    right--;
}

// Sliding window
int windowSum = 0;
for (int i = 0; i < k; i++) {
    windowSum += arr[i];
}
for (int i = k; i < arr.length; i++) {
    windowSum = windowSum - arr[i-k] + arr[i];
    // Process windowSum
}

String Manipulation

Strings are essentially character arrays. Common operations include:

Length: str.length()
Character access: str.charAt(i)
Substring: str.substring(start, end)
Character array: str.toCharArray()
String building: StringBuilder

String Examples

// Palindrome check
boolean isPalindrome(String s) {
    int left = 0, right = s.length() - 1;
    while (left < right) {
        if (s.charAt(left) != s.charAt(right)) {
            return false;
        }
        left++;
        right--;
    }
    return true;
}

// String reversal
String reverse(String s) {
    StringBuilder sb = new StringBuilder(s);
    return sb.reverse().toString();
}

// Character frequency
int[] getFrequency(String s) {
    int[] freq = new int[26];
    for (char c : s.toCharArray()) {
        freq[c - 'a']++;
    }
    return freq;
}

Key Techniques

Two Pointers: For palindrome, two sum, container problems
Sliding Window: For substring, subarray problems
Prefix Sum: For range sum queries
Kadane's Algorithm: For maximum subarray sum
Dutch National Flag: For array partitioning

Open full interactive app