Count the number of repeat integers in a sorted array.

count-array-dupes-sorted.js

function first(arr, target, left, right, found=-1) {
    const mid = Math.floor((left+right)/2);
    found = arr[mid] === target ? mid : found;
    if(right > left) {
        if(arr[mid] >= target) {
            return first(arr, target, left, mid-1, found);
        }else if(arr[mid] < target) {
            return first(arr, target, mid+1, right, found);
        }
    }
    return found;
}

function last(arr, target, left, right, found=-1) {
    const mid = Math.floor((left+right)/2);
    found = arr[mid] === target ? mid : found;
    if(right > left) {
        if(arr[mid] > target) {
            return last(arr, target, left, mid-1, found);
        }else if(arr[mid] <= target) {
            return last(arr, target, mid+1, right, found);
        }
    }
    return found;
}

function countDupes(arr, target) {
    const left = first(arr, target, 0, arr.length);
    const right = last(arr, target, left, arr.length);
    const count = left >= 0 ? right-(left-1) : 0;
    
    return count;
}



const arr = [1,2,3,3,3,3,4,5,6,7,7,7,7,9,9];
// const arr = [1,2,2,2,3];

new Set(arr).forEach((value) => {
    const dupes = countDupes(arr, value);
    console.log(`value: ${value}, dupes: ${dupes}`)
});