deduplicateLetters

let letters='abbace'
function deduplicateLetters(letters){
    let stack=[]
    for(i of letters){
        let prev=stack.pop()
        if(prev!=i){
            stack.push(prev)
            stack.push(i)
        }
    }
    return stack.join('')
}
console.log(deduplicateLetters(letters));