import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static File _f;
void file_io(string fn) { _f = File(fn, "r"); }
static string[] s_rd;
T _RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T _RD(T = long)(File f) { while(!s_rd.length) s_rd = f.readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T[] _RDA(T = long)(T fix = 0) { auto r = readln.chomp.split.to!(T[]); r[] += fix; return r; }
T[] _RDA(T = long)(File f, T fix = 0) { auto r = f.readln.chomp.split.to!(T[]); r[] += fix; return r; }
T RD(T = long)() { if (_f.isOpen) return _RD!T(_f); else return _RD!T; }
T[] RDA(T = long)(T fix = 0) { if (_f.isOpen)  return _RDA!T(_f, fix); else return _RDA!T(fix); }

size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
long lcm(long x, long y) { return x * (y / gcd(x, y)); }

//long mod = 10^^9 + 7;
long mod = 998244353;
//long mod = 1_000_003;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }

void main()
{
	auto k = RD!int;
	auto ans = new int[2][][](k);
	foreach (i; 0..k)
	{
		auto n = RD!int;
		auto s = RD!(char[]);
		auto t = RD!(char[]);
		auto cnt = new int[](26);
		foreach (j; 0..n)
		{
			auto c1 = s[j] - 'a';
			auto c2 = t[j] - 'a';
			++cnt[c1];
			++cnt[c2];
		}
		bool ok = true;
		foreach (e; cnt)
		{
			if (e % 2 == 1)
			{
				ok = false;
				break;
			}
		}
		if (!ok) continue;
		foreach (j; 0..n)
		{
			if (s[j] != t[j])
			{
				int pos = -1;
				foreach (l; j+1..n)
				{
					if (t[l] == t[j])
					{
						pos = l;
						break;
					}
				}
				if (pos == -1)
				{
					foreach (l; j+1..n)
					{
						if (s[l] == t[j])
						{
							pos = l;
							break;
						}
					}
					ans[i] ~= [pos, pos];
					ans[i] ~= [j, pos];
					swap(s[pos], t[pos]);
					swap(s[j], t[pos]);
				}
				else
				{
					ans[i] ~= [j, pos];
					swap(s[j], t[pos]);
				}
			}
		}
	}

	foreach (e; ans)
	{
		if (e.empty)
			writeln("NO");
		else
		{
			writeln("YES");
			writeln(e.length);
			foreach (ee; e)
				writeln(ee[0]+1, " ", ee[1]+1);
		}
	}
	stdout.flush();
	debug readln();
}