2190. Most Frequent Number Following Key In an Array 

Description

You are given a 0-indexed integer array nums. You are also given an integer key, which is present in nums.

For every unique integer target in nums, count the number of times target immediately follows an occurrence of key in nums. In other words, count the number of indices i such that:

0 <= i <= nums.length - 2,
nums[i] == key and,
nums[i + 1] == target.

Return the target with the maximum count. The test cases will be generated such that the target with maximum count is unique.

Example 1:

Input: nums = [1,100,200,1,100], key = 1
Output: 100
Explanation: For target = 100, there are 2 occurrences at indices 1 and 4 which follow an occurrence of key.
No other integers follow an occurrence of key, so we return 100.

Example 2:

Input: nums = [2,2,2,2,3], key = 2
Output: 2
Explanation: For target = 2, there are 3 occurrences at indices 1, 2, and 3 which follow an occurrence of key.
For target = 3, there is only one occurrence at index 4 which follows an occurrence of key.
target = 2 has the maximum number of occurrences following an occurrence of key, so we return 2.

Constraints:

2 <= nums.length <= 1000
1 <= nums[i] <= 1000
The test cases will be generated such that the answer is unique.

Solutions

Solution 1: Traversal and Counting

We use a hash table or an array $\textit{cnt}$ to record the number of occurrences of each $\textit{target}$, and use a variable $\textit{mx}$ to maintain the maximum number of occurrences of $\textit{target}$. Initially, $\textit{mx} = 0$.

Traverse the array $\textit{nums}$. If $\textit{nums}[i] = \textit{key}$, increment the count of $\textit{nums}[i + 1]$ in $\textit{cnt}[\textit{nums}[i + 1]]$. If $\textit{mx} \lt \textit{cnt}[\textit{nums}[i + 1]]$, update $\textit{mx} = \textit{cnt}[\textit{nums}[i + 1]]$ and update the answer $\textit{ans} = \textit{nums}[i + 1]$.

After the traversal, return the answer $\textit{ans}$.

The time complexity is $O(n)$, and the space complexity is $O(M)$. Here, $n$ and $M$ are the length of the array $\textit{nums}$ and the maximum value of the elements in the array $\textit{nums}$, respectively.

Python3

class Solution:
    def mostFrequent(self, nums: List[int], key: int) -> int:
        cnt = Counter()
        ans = mx = 0
        for a, b in pairwise(nums):
            if a == key:
                cnt[b] += 1
                if mx < cnt[b]:
                    mx = cnt[b]
                    ans = b
        return ans

Java

class Solution {
    public int mostFrequent(int[] nums, int key) {
        int[] cnt = new int[1001];
        int ans = 0, mx = 0;
        for (int i = 0; i < nums.length - 1; ++i) {
            if (nums[i] == key) {
                if (mx < ++cnt[nums[i + 1]]) {
                    mx = cnt[nums[i + 1]];
                    ans = nums[i + 1];
                }
            }
        }
        return ans;
    }
}

C++

class Solution {
public:
    int mostFrequent(vector<int>& nums, int key) {
        int cnt[1001]{};
        int ans = 0, mx = 0;
        for (int i = 0; i < nums.size() - 1; ++i) {
            if (nums[i] == key) {
                if (mx < ++cnt[nums[i + 1]]) {
                    mx = cnt[nums[i + 1]];
                    ans = nums[i + 1];
                }
            }
        }
        return ans;
    }
};

Go

func mostFrequent(nums []int, key int) (ans int) {
	cnt := [1001]int{}
	mx := 0
	for i, x := range nums[1:] {
		if nums[i] == key {
			cnt[x]++
			if mx < cnt[x] {
				mx = cnt[x]
				ans = x
			}
		}
	}
	return
}

TypeScript

function mostFrequent(nums: number[], key: number): number {
    const cnt: number[] = Array(Math.max(...nums) + 1).fill(0);
    let [ans, mx] = [0, 0];
    for (let i = 0; i < nums.length - 1; ++i) {
        if (nums[i] === key) {
            if (mx < ++cnt[nums[i + 1]]) {
                mx = cnt[nums[i + 1]];
                ans = nums[i + 1];
            }
        }
    }
    return ans;
}

JavaScript

/**
 * @param {number[]} nums
 * @param {number} key
 * @return {number}
 */
var mostFrequent = function (nums, key) {
    const cnt = Array(Math.max(...nums) + 1).fill(0);
    let [ans, mx] = [0, 0];
    for (let i = 0; i < nums.length - 1; ++i) {
        if (nums[i] === key) {
            if (mx < ++cnt[nums[i + 1]]) {
                mx = cnt[nums[i + 1]];
                ans = nums[i + 1];
            }
        }
    }
    return ans;
};

PHP

class Solution {
    function mostFrequent($nums, $key) {
        $cnt = array_fill(0, max($nums) + 1, 0);
        $ans = 0;
        $mx = 0;
        for ($i = 0; $i < count($nums) - 1; ++$i) {
            if ($nums[$i] === $key) {
                $cnt[$nums[$i + 1]]++;
                if ($mx < $cnt[$nums[$i + 1]]) {
                    $mx = $cnt[$nums[$i + 1]];
                    $ans = $nums[$i + 1];
                }
            }
        }
        return $ans;
    }
}