Coding Problems

See also: Interview Questions - Coding Problems for interview-focused coding problems and solutions.

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Print 1..N without a loop (recursion)

static void printNos(int n) {
    if (n > 0) {
        printNos(n - 1);
        System.out.print(n + " ");
    }
}
// printNos(100);

Palindrome without a loop

Recursion:

public boolean isPalindromeRecursive(String text) {
    String clean = text.replaceAll("\\s+", "").toLowerCase();
    return recursivePalindrome(clean, 0, clean.length() - 1);
}
private boolean recursivePalindrome(String s, int fwd, int bwd) {
    if (fwd >= bwd) return true;
    if (s.charAt(fwd) != s.charAt(bwd)) return false;
    return recursivePalindrome(s, fwd + 1, bwd - 1);
}

Stream API:

public boolean isPalindrome(String text) {
    String t = text.replaceAll("\\s+", "").toLowerCase();
    return IntStream.range(0, t.length() / 2)
        .noneMatch(i -> t.charAt(i) != t.charAt(t.length() - i - 1));
}

Max subarray sum (Kadane's algorithm)

public static int maxSubArray(int[] nums) {
    int maxSum = Integer.MIN_VALUE, curr = 0;
    for (int n : nums) {
        curr += n;
        maxSum = Math.max(maxSum, curr);
        if (curr < 0) curr = 0;
    }
    return maxSum;
}
// [-2,1,-3,4,-1,2,1,-5,4] -> 6  (subarray [4,-1,2,1])

O(n) time, O(1) space — an iterative DP.

More to add

See Backlog: sort students by mark + rank, conversions, bitwise tricks.