Traversal

Snake

Python

R, C = len(matrix), len(matrix[0])
for r in range(R):
    if r % 2 == 0:
        # L2R
        for c in range(C):
            print(matrix[r][c])
    else:
        # R2L
        for c in range(C - 1, -1, -1):
            print(matrix[r][c])

Spiral

Python

l, r = 0, len(matrix[0]) - 1 # left, right
t, b = 0, len(matrix) - 1 # top, bottom
while l <= r and t <= b:
    for i in range(l, r + 1):
        print(matrix[t][i])
    t += 1
    for i in range(t, b + 1):
        print(matrix[i][r])
    r -= 1
    if t <= b:
        for i in range(r, l - 1, -1):
            print(matrix[b][i])
        b -= 1
    if l <= r:
        for i in range(b, t - 1, -1):
            print(matrix[i][l])
        l += 1