function findint(input) {
    let bits = (1 << input.length) - 1;

    console.log('s:', (bits).toString(2));

    for(let i=0; i<input.length; i++) {
        let curr = input[i];

        if(curr <= input.length && curr > 0) {
            bits = bits ^ (1 << (curr-1))
        }

        console.log((bits).toString(2));
    }

    bits &= -bits;
    console.log('e:', (bits).toString(2));
    return Math.log2(bits) + 1;
}

// len = 5 : 1 2 3 4 5

console.log(findint([1, 2, 0])) // 3
console.log(findint([3, 4, -1, 1])) // 2