/**
 * @param {character[]} s
 * @return {void} Do not return anything, modify s in-place instead.
 */
var reverseString = function(s) {
    let start=0;
    let end=s.length-1;
    while(start<end){
        [s[start],s[end]]=[s[end],s[start]];
        start++;
        end--;
    }
    return s;

};

双指针，一头一尾互换值即可