GFG Shortest XY distance in Grid

Give a N*M grid of characters 'O', 'X', and 'Y'. Find the minimum Manhattan distance between a X and a Y.

Manhattan Distance : | row_index_x - row_index_y | + | column_index_x - column_index_y |

Example 1:

Input:

N = 4, M = 4

grid = {{X, O, O, O}

{O, Y, O, Y}

{X, X, O, O}

{O, Y, O, O}}

Output:

1

Explanation:

{{X, O, O, O}

{O, Y, O, Y}

{X, X, O, O}

{O, Y, O, O}}

The shortest X-Y distance in the grid is 1.

One possible such X and Y are marked in bold

in the above grid.

Example 2:

Input:

N = 3, M = 3

grid = {{X, X, O}

{O, O, Y}

{Y, O, O}}

Output :

2

Explanation:

{{X, X, O}

{O, O, Y}

{Y, O, O}}

The shortest X-Y distance in the grid is 2.

One possible such X and Y are marked in bold

in the above grid.

Your Task:  

You don't need to read input or print anything. Your task is to complete the function

shortestXYDist() which takes two integers N, and M and an 2D list of size N*M as input and

returns the shortest Manhattan Distance between a X and a Y.

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BFS求最短路问题，

做过好几道类似的题目了，所以能直接自己做出来了。