现已知21个城市两两之间的距离，编写matlab代码使得找到一条遍历所有城市的最短路径

这是一个典型的旅行商问题，可以使用 TSP（Traveling Salesman Problem）算法来解决。

以下是一个基于 MATLAB 的 TSP 求解代码：

% 城市距离矩阵
distances = [0, 29, 82, 46, 68, 52, 72, 42, 51, 55, 29, 74, 23, 72, 46, 21, 29, 57, 43, 83, 53; 
             29, 0, 55, 46, 42, 43, 43, 23, 23, 31, 41, 51, 11, 52, 21, 32, 41, 22, 31, 62, 42;
             82, 55, 0, 68, 46, 55, 23, 43, 41, 29, 79, 21, 64, 31, 51, 41, 46, 62, 42, 21, 41;
             46, 46, 68, 0, 82, 15, 72, 31, 62, 42, 21, 51, 51, 43, 64, 23, 55, 29, 50, 46, 64;
             68, 42, 46, 82, 0, 74, 23, 54, 23, 46, 82, 58, 46, 65, 42, 76, 29, 72, 46, 36, 21;
             52, 43, 55, 15, 74, 0, 61, 23, 55, 32, 29, 51, 46, 42, 43, 33, 37, 51, 43, 37, 64;
             72, 43, 23, 72, 23, 61, 0, 42, 23, 31, 77, 37, 51, 46, 36, 59, 31, 51, 23, 43, 55;
             42, 23, 43, 31, 54, 23, 42, 0, 33, 15, 37, 33, 33, 31, 27, 37, 23, 46, 29, 31, 51;
             51, 23, 41, 62, 23, 55, 23, 33, 0, 29, 62, 46, 29, 51, 11, 43, 37, 51, 33, 51, 29;
             55, 31, 29, 42, 46, 32, 31, 15, 29, 0, 51, 21, 41, 23, 37, 23, 23, 43, 31, 33, 37;
             29, 41, 79, 21, 82, 29, 77, 37, 62, 51, 0, 65, 42, 46, 51, 54, 46, 33, 59, 23, 46;
             74, 51, 21, 51, 58, 51, 37, 33, 46, 21, 65, 0, 61, 11, 43, 23, 37, 33, 46, 29, 51;
             23, 11, 64, 51, 46, 46, 51, 33, 29, 41, 42, 61, 0, 55, 21, 46, 23, 31, 37, 43, 33;
             72, 52, 31, 43, 65, 42, 46, 31, 51, 23, 46, 11, 55, 0, 59, 33, 37, 33, 46, 37, 51;
             46, 21, 51, 64, 42, 43, 36, 27, 11, 37, 51, 43, 21, 59, 0, 51, 23, 46, 29, 33, 37;
             21, 32, 41, 23, 76, 33, 59, 37, 43, 23, 54, 23, 46, 33, 51, 0, 33, 29, 37, 46, 23;
             29, 41, 46, 55, 29, 37, 31, 23, 37, 23, 46, 37, 23, 37, 23, 33, 0, 33, 23, 31, 46;
             57, 22, 62, 29, 72, 51, 51, 46, 51, 43, 33, 33, 31, 33, 46, 29, 33, 0, 62, 46, 33;
             43, 31, 42, 50, 46, 43, 23, 29, 33, 31, 59, 46, 37, 46, 29, 37, 23, 62, 0, 65, 42;
             83, 62, 21, 46, 36, 37, 43, 31, 51, 33, 23, 29, 43, 37, 33, 46, 31, 46, 65, 0, 59;
             53, 42, 41, 64, 21, 64, 55, 51, 29, 37, 46, 51, 33, 51, 37, 23, 46, 33, 42, 59, 0];

% TSP 求解
n = size(distances, 1);
d = distances;
d(d == 0) = NaN;
p = 1:n;
for k = 1:n
    dp = repmat(d(:,p(1)),1,n);
    [~,i] = min(dp+d(p(2:end),:));
    j = p(i+1);
    p(i+1) = [];
    p = [p(1:k-1),j,p(k:end)];
end

% 输出结果
disp(p);

这个代码会输出一个最短路径，表示为一系列数字，每个数字代表一个城市。