DSA using JavaScript

Time Complexity: O(n^2)
Space Complexity: O(1)

Binary Search

function findMissingBruteForce(arr) {
    for (let i = 1; i <= arr.length + 1; i++) {
        if (!arr.includes(i)) {
            return i;
        }
    }
    return -1;
}

const arr = [1, 3, 4, 5];
console.log(findMissingBruteForce(arr));  // Output: 2

function findMissingEfficient(arr) {
    let left = 0, right = arr.length - 1;

    while (left <= right) {
        const mid = Math.floor((left + right) / 2);
        
        if (arr[mid] === mid + 1) {
            left = mid + 1;
        } else {
            right = mid - 1;
        }
    }
    
    return left + 1;
}

const arr = [1, 3, 4, 5];
console.log(findMissingEfficient(arr));  // Output: 2