function findConsecutiveRuns (arr) {
    const runs = [];
    for (let i = 0; i < arr.length; i++) {
        const curr = arr[i];
        const next = arr[i+1];
        const last = arr[i+2];
        if ( Math.abs(next - curr) == 1 && Math.abs(curr - last) == 2 ) {
            runs.push(i);
        }
    }
    return runs.length ? runs : null;
}