163. Missing Ranges

Given a sorted integer array where the range of elements are [lower, upper] inclusive, return its missing ranges.

For example, given [0, 1, 3, 50, 75], lower = 0 and upper = 99, return ["2", "4->49", "51->74", "76->99"].

function findMissingRanges(vals, start, end) {
    var ranges = [];
    var prev = start - 1;
    for (var i = 0; i <= vals.length; i++) {
        var curr = (i == vals.length) ? end + 1 : vals[i];
        if (curr - prev >= 2) {
            ranges.push(getRange(prev + 1, curr - 1));
        }
        prev = curr;
    }
    return ranges;
}

function getRange(from, to) {
    return (from == to) ? String(from) : from + "->" + to;
}