Leaders array JavaScript

An element in an array is a leader if it is greater than all elements on its right side. The rightmost element is always a leader since there are no elements to its right.

For example, in the array [23, 55, 2, 56, 3, 6, 7, 1]:

• 56 is a leader (greater than 3, 6, 7, 1)
• 7 is a leader (greater than 1)
• 1 is a leader (rightmost element)

Input: [23, 55, 2, 56, 3, 6, 7, 1]
Output: [56, 7, 1]

Using reduceRight() Method

The most efficient approach is to traverse the array from right to left, keeping track of the maximum element seen so far:

const arr = [23, 55, 2, 56, 3, 6, 7, 1];

const leaderArray = arr => {
    const creds = arr.reduceRight((acc, val) => {
        let { max, res } = acc;
        if(val > max) {
            res.unshift(val);
            max = val;
        }
        return { max, res };
    }, {
        max: -Infinity,
        res: []
    });
    return creds.res;
};

console.log(leaderArray(arr));

[56, 7, 1]

Using Simple Loop Method

A more straightforward approach using a for loop:

const findLeaders = arr => {
    const leaders = [];
    const n = arr.length;
    
    // Rightmost element is always a leader
    let maxFromRight = arr[n - 1];
    leaders.push(maxFromRight);
    
    // Traverse from right to left
    for (let i = n - 2; i >= 0; i--) {
        if (arr[i] > maxFromRight) {
            maxFromRight = arr[i];
            leaders.push(arr[i]);
        }
    }
    
    // Reverse to get leaders in original order
    return leaders.reverse();
};

const array = [16, 17, 4, 3, 5, 2];
console.log(findLeaders(array));

[17, 5, 2]

How It Works

Both methods use the same logic:

1. Start from the rightmost element (always a leader)
2. Move left, comparing each element with the maximum seen so far
3. If current element is greater than the maximum, it's a leader
4. Update the maximum and continue

Comparison

Conclusion

Finding leader elements requires a single right-to-left traversal with O(n) time complexity. Use reduceRight() for functional programming style or a simple loop for better readability.