Answer
156423, 165432, 231456, 231465, 234561, 314562, 432561, 435612, 541236, 543216, 654312, 654321
Work Step by Step
Placing permutations of (1,2,3,4,5,6) in lexicographic order :
156423, 165432, 231456, 231465, 234561, 314562, 432561, 435612, 541236, 543216, 654312, 654321