Time complexity is O(m*n)
題目要求找出每個元素離最近的0的距離，
1. 找每個1，然後去找最近的0，特殊案例就是大多都是1，只有一個0
2. 找每個0，然後去找0附近的1，特殊案例就是大多都是0，只有一個1
因為是一個一個往外擴散，所以適用BFS。
“return the distance of the nearest 0 for each cell”
consider 1. find each 1, then find the nearest 0 and count the distance. The worst case is [[1,1,1],[1,1,1],[1,0,1]].
consider 2. find each 0, depend on the coordinate of 0 to search 1 and calculate the level. The worst case is [[0,0,0],[0,0,0],[0,0,1]].
level by level to search is qualify BFS.