离散化

    n = read();
	for (int i = 1; i <= n; ++ i) {
		a[i] = read();
		b[i] = a[i];
	}
	stable_sort(b + 1, b + 1 + n);
	len = unique(b + 1, b + 1 + n) - (b + 1);
	for (int i = 1; i <= n; ++ i) {
		a[i] = lower_bound(b + 1, b + 1 + len, a[i]) - b;
	}
    //a[i]是b[i]离散化后的数组