For example we have:

["p", "r", "e", "f", "e", "t", " ", "m", "a", "k", "e", " ", "p", "r", "a", "t", "i", " c", "e"]

We want to get:

["p", "r", "a", "t", "i", " c", "e", " ", "m", "a", "k", "e", " ", "p", "r", "e", "f", "e", "t"]

 

Requirement:

　　You can only do in array swap, you cannot create a new array.

 

The way to do it:

　　1. Reverse whole string to get:

["e", " c", "i", "t", "a", "r", "p", " ", "e", "k", "a", "m", " ", "t", "e", "f", "e", "r", "p"]

　　2. Then we reverse each word to get final result:

["p", "r", "a", "t", "i", " c", "e", " ", "m", "a", "k", "e", " ", "p", "r", "e", "f", "e", "t"]

 

So the main function should looks like this:

let data = [  "p","r","e","f","e","t"," ",  "m","a","k","e"," ",  "p","r","a","t","i","c","e"];function main(_data) {  let data = _data.slice();  reverseWholeString(data);  reverseWords(data, 0);  return data;}console.log(main(data));

 

The reverseWholeString function would be:

function reverseWholeString(data) {  let start = 0,    end = data.length - 1;  reverseChars(data, start, end);}function reverseChars(data, start, end) {  while (start < end) {    [data[start], data[end]] = [data[end], data[start]];    start++;    end--;  }}

 

reverseWords function would be:

function reverseWords(data, start) {  let index = findEmptyIndex(data, start);  let end = index - 1;  while (index !== -1) {    reverseChars(data, start, end);    start = index + 1;    index = findEmptyIndex(data, start);    end = index - 1;  }  reverseChars(data, start, data.length - 1);}function findEmptyIndex(data, start) {  let index;  for (let i = start; i < data.length; i++) {    if (data[i] === " ") {      index = i;      break;    } else {      index = -1;    }  }  return index;}

 

------------

Full code:

let data = [  "p",  "r",  "e",  "f",  "e",  "t",  " ",  "m",  "a",  "k",  "e",  " ",  "p",  "r",  "a",  "t",  "i",  " c",  "e"];function reverseWholeString(data) {  let start = 0,    end = data.length - 1;  reverseChars(data, start, end);}function reverseChars(data, start, end) {  while (start < end) {    [data[start], data[end]] = [data[end], data[start]];    start++;    end--;  }}function findEmptyIndex(data, start) {  let index;  for (let i = start; i < data.length; i++) {    if (data[i] === " ") {      index = i;      break;    } else {      index = -1;    }  }  return index;}function reverseWords(data, start) {  let index = findEmptyIndex(data, start);  let end = index - 1;  while (index !== -1) {    reverseChars(data, start, end);    start = index + 1;    index = findEmptyIndex(data, start);    end = index - 1;  }  reverseChars(data, start, data.length - 1);}function main(_data) {  let data = _data.slice();  reverseWholeString(data);  reverseWords(data, 0);  return data;}console.log(main(data));