Question

Write a function that removes all duplicate letters from given string?
function solve( a ){ //write your code here }
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Note: Duplicate letters must be removed from start.

Input: "book"
Output: "bok"

Input: "cat"
Output: "cat"

Input: "jnqvzv"
Output: "jnqzv"

Input: "ijilktba"
Output: "jilktba"