X1662: insertUnderscore

Description

Given a string, compute recursively a new string where all the adjacent chars are now separated by an underscore ("_"). So given "hello", return "h_e_l_l_o".

Constraints

You cannot use the following methods from the String class: indexOf, lastIndexOf, contains, endsWith, startsWith, matches, replace, nor replaceAll.

Documentation

The String documentation is available online.

Examples:

insertUnderscore("abc") -> "a_b_c"

insertUnderscore("a") -> "a"

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