LeetCode #1985 — MEDIUM

Find the Kth Largest Integer in the Array

Move from brute-force thinking to an efficient approach using array strategy.

The Problem

Problem Statement

You are given an array of strings nums and an integer k. Each string in nums represents an integer without leading zeros.

Return the string that represents the k^th largest integer in nums.

Note: Duplicate numbers should be counted distinctly. For example, if nums is ["1","2","2"], "2" is the first largest integer, "2" is the second-largest integer, and "1" is the third-largest integer.

Example 1:

Input: nums = ["3","6","7","10"], k = 4
Output: "3"
Explanation:
The numbers in nums sorted in non-decreasing order are ["3","6","7","10"].
The 4^th largest integer in nums is "3".

Example 2:

Input: nums = ["2","21","12","1"], k = 3
Output: "2"
Explanation:
The numbers in nums sorted in non-decreasing order are ["1","2","12","21"].
The 3^rd largest integer in nums is "2".

Example 3:

Input: nums = ["0","0"], k = 2
Output: "0"
Explanation:
The numbers in nums sorted in non-decreasing order are ["0","0"].
The 2^nd largest integer in nums is "0".

Constraints:

1 <= k <= nums.length <= 10⁴
1 <= nums[i].length <= 100
nums[i] consists of only digits.
nums[i] will not have any leading zeros.

Step 01

Brute Force Baseline

Problem summary: You are given an array of strings nums and an integer k. Each string in nums represents an integer without leading zeros. Return the string that represents the kth largest integer in nums. Note: Duplicate numbers should be counted distinctly. For example, if nums is ["1","2","2"], "2" is the first largest integer, "2" is the second-largest integer, and "1" is the third-largest integer.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Array

Example 1

["3","6","7","10"]
4

Example 2

["2","21","12","1"]
3

Example 3

["0","0"]
2

Core Insight

What unlocks the optimal approach

If two numbers have different lengths, which one will be larger?
The longer number is the larger number.
If two numbers have the same length, which one will be larger?
Compare the two numbers starting from the most significant digit. Once you have found the first digit that differs, the one with the larger digit is the larger number.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #1985: Find the Kth Largest Integer in the Array
class Solution {
    public String kthLargestNumber(String[] nums, int k) {
        Arrays.sort(
            nums, (a, b) -> a.length() == b.length() ? b.compareTo(a) : b.length() - a.length());
        return nums[k - 1];
    }
}

// Accepted solution for LeetCode #1985: Find the Kth Largest Integer in the Array
func kthLargestNumber(nums []string, k int) string {
	sort.Slice(nums, func(i, j int) bool {
		a, b := nums[i], nums[j]
		if len(a) == len(b) {
			return a > b
		}
		return len(a) > len(b)
	})
	return nums[k-1]
}

# Accepted solution for LeetCode #1985: Find the Kth Largest Integer in the Array
class Solution:
    def kthLargestNumber(self, nums: List[str], k: int) -> str:
        return nlargest(k, nums, key=lambda x: int(x))[k - 1]

// Accepted solution for LeetCode #1985: Find the Kth Largest Integer in the Array
/**
 * [1985] Find the Kth Largest Integer in the Array
 *
 * You are given an array of strings nums and an integer k. Each string in nums represents an integer without leading zeros.
 * Return the string that represents the k^th largest integer in nums.
 * Note: Duplicate numbers should be counted distinctly. For example, if nums is ["1","2","2"], "2" is the first largest integer, "2" is the second-largest integer, and "1" is the third-largest integer.
 *
 * Example 1:
 *
 * Input: nums = ["3","6","7","10"], k = 4
 * Output: "3"
 * Explanation:
 * The numbers in nums sorted in non-decreasing order are ["3","6","7","10"].
 * The 4^th largest integer in nums is "3".
 *
 * Example 2:
]*
 * Input: nums = ["2","21","12","1"], k = 3
 * Output: "2"
 * Explanation:
 * The numbers in nums sorted in non-decreasing order are ["1","2","12","21"].
 * The 3^rd largest integer in nums is "2".
 *
 * Example 3:
 *
 * Input: nums = ["0","0"], k = 2
 * Output: "0"
 * Explanation:
 * The numbers in nums sorted in non-decreasing order are ["0","0"].
 * The 2^nd largest integer in nums is "0".
 *
 *
 * Constraints:
 *
 * 	1 <= k <= nums.length <= 10^4
 * 	1 <= nums[i].length <= 100
 * 	nums[i] consists of only digits.
 * 	nums[i] will not have any leading zeros.
 *
 */
pub struct Solution {}

// problem: https://leetcode.com/problems/find-the-kth-largest-integer-in-the-array/
// discuss: https://leetcode.com/problems/find-the-kth-largest-integer-in-the-array/discuss/?currentPage=1&orderBy=most_votes&query=

// submission codes start here

impl Solution {
    pub fn kth_largest_number(nums: Vec<String>, k: i32) -> String {
        let mut nums = nums;
        let k = (k - 1) as usize;

        nums.select_nth_unstable_by(k, |a, b| (b.len()).cmp(&a.len()).then_with(|| b.cmp(&a)));

        nums[k].clone()
    }
}

// submission codes end

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_1985_example_1() {
        let nums = vec_string!["3", "6", "7", "10"];
        let k = 4;

        let result = "3".to_string();

        assert_eq!(Solution::kth_largest_number(nums, k), result);
    }

    #[test]
    fn test_1985_example_2() {
        let nums = vec_string!["2", "21", "12", "1"];
        let k = 3;

        let result = "2".to_string();

        assert_eq!(Solution::kth_largest_number(nums, k), result);
    }

    #[test]
    fn test_1985_example_3() {
        let nums = vec_string!["0", "0"];
        let k = 2;

        let result = "0".to_string();

        assert_eq!(Solution::kth_largest_number(nums, k), result);
    }
}

// Accepted solution for LeetCode #1985: Find the Kth Largest Integer in the Array
// Auto-generated TypeScript example from java.
function exampleSolution(): void {
}
// Reference (java):
// // Accepted solution for LeetCode #1985: Find the Kth Largest Integer in the Array
// class Solution {
//     public String kthLargestNumber(String[] nums, int k) {
//         Arrays.sort(
//             nums, (a, b) -> a.length() == b.length() ? b.compareTo(a) : b.length() - a.length());
//         return nums[k - 1];
//     }
// }

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n)

Space

O(1)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED

O(n) time

O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.