題目來源：劍指 Offer 38. 字串的排列

輸入一個字串，打印出該字串中字元的所有排列。

你可以以任意順序返回這個字串陣列，但裡面不能有重複元素。

方法一：回溯

/**
 * @param {string} s
 * @return {string[]}
 */
var permutation = function(s) {
    let n = s.length;
    let perm = [];
    let res = [];
    let vis = [];
    let arr = Array.from(s).sort();
    var backTrack = function
 
 (i) {
        if(i === n){
            res.push( perm.join(''));
            return ;
        }
        for(let j=0;j<n;j++){
            if(vis[j] || (j>0 && !vis[j-1] && arr[j-1] === arr[j]) ){
                continue;
            }
            perm.push(arr[j]);
            vis[j] 
 
= true;
            backTrack(i+1);
            perm.pop();
            vis[j] = false;
        }
    }
    backTrack(0);
    return res;
};

方法二：下一個排列

/**
 * @param {string} s
 * @return {string[]}
 */
var permutation = function
 
(s) {
    let n = s.length;
    let res = [];
    let arr = Array.from(s).sort();  
    
    var nextPermutation = (a)=>{
        let i = n - 2;
        while(i>=0 && a[i] >= a[i+1]){
            i--;
        }
        if(i<0){
            return false;
        }
        let j = n - 1;
        while(j>=0 && a[i]>=a[j]){
            j--;
        }
        [a[i], a[j]] = [a[j], a[i]];
        arr = [].concat(a.slice(0, i+1), a.slice(i+1).reverse());
        return true;
    }
    do{
        res.push(arr.join(''))
    }while(nextPermutation(arr))
    return res;
};

示例:

輸入：s = "abc"
輸出：["abc","acb","bac","bca","cab","cba"]

限制：

1 <= s 的長度 <= 8