java写着写着，吐了，后来想用 js 写，虽然比java还要贫瘠，但至少可以随性些。 

    /// <summary>
    /// Map orignal indexes to sorted indexes, eg: SI2I([2,5,9,4,6,7])=[0(0,2),3(1,4),1(2,5),4(3,6),5(4,7),2(5,9)].
    /// </summary>
    /// <returns></returns>
    public static int[] SI2I<T>(this List<T> s, Func<T, int> by) {
        var r = new List<int>();
        var t = new List<T>(s);
        while (r.Count < s.Count) {
            var f = 0;
            var v = by(t[0]);
            for (var i = 1; i < t.Count; i++) {
                var n = by(t[i]);
                if (n < v) {
                    f = i;
                    v = n;
                }
            }
            var ts = t[f];
            var ii = s.IndexOf(ts);
            while (r.Contains(ii)) {
                ii = s.IndexOf(ts, ii + 1);
            }
            r.Add(ii);
            t.RemoveAt(f);
        }
        return r.ToArray();
    }

    /// <summary>
    /// Map sorted indexes to orignal indexes, see <see cref="SI2I{T}(List{T}, Func{T, int})"/>.
    /// </summary>
    public static int[] I2SI<T>(this List<T> s, Func<T, int> by) {
        return SI2I(SI2I(s, by).ToList(), i => i);
    }
}

获得排序后的下标与原始下标关系