Combinations VS Permutation

Comination

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:
[

[1,2],

[1,3],

[1,4],

[2,4],
[2,3],
[3,4],
]

Solution

It's a way to simulate the process of getting 2 elements out of 4,

1. In round one, select 1 from 1...4 as the first number, you have [1]

2. next recursion, select one element from 2...n, you have [1,2], [1, 3], [1,4]

3 back to round one, select 2 from 2...4 as the first number, you have [2]

4. next recursion, select one element from 3..4, you have [2.3], [2,4]

......

Permutation

Get all permutations of a string

Solution

Swap the first element with all the other elements and permute on the rest of the string