e=enumerate
f=lambda M:[[r[n-i]for i,r in e(M)if-1<n-i<len(r)][::-1]for n,_ in e(M[1:]+M[0])]
for M in [[1]], [[1,2],[3,4]], [[1,2,3],[4,5,6]], [[11,2,5],[3,99,3],[4,8,15],[16,23,42]]:
print(f(M))
ZT1lbnVtZXJhdGUKZj1sYW1iZGEgTTpbW3Jbbi1pXWZvciBpLHIgaW4gZShNKWlmLTE8bi1pPGxlbihyKV1bOjotMV1mb3IgbixfIGluIGUoTVsxOl0rTVswXSldCgpmb3IgTSBpbiBbWzFdXSwgW1sxLDJdLFszLDRdXSwgW1sxLDIsM10sWzQsNSw2XV0sIFtbMTEsMiw1XSxbMyw5OSwzXSxbNCw4LDE1XSxbMTYsMjMsNDJdXToKCXByaW50KGYoTSkp
[[1]]
[[1], [3, 2], [4]]
[[1], [4, 2], [5, 3], [6]]
[[11], [3, 2], [4, 99, 5], [16, 8, 3], [23, 15], [42]]