Double Character Swap

Write a function to replace all instances of character c1 with character c2 and vice versa.

Examples

double_swap("aabbccc", "a", "b") ➞ "bbaaccc"

double_swap("random w#rds writt&n h&r&", "#", "&")
➞ "random w&rds writt#n h#r#"

double_swap("128 895 556 788 999", "8", "9")
➞ "129 985 556 799 888"

Notes

Both characters will show up at least once in the string.