为什么我的变量引用全局范围变量？

Question

I tried to write a function that finds all possible permutations given an array of numbers. Here is my code.

var perm = [];
var usedchar = [];

function findAllPermutations(arr) {

  for (var i = 0; i < arr.length; i++) {
    var x = arr.splice(i,1)[0];
    usedchar.push(x);

    if (usedchar.length == 3) {
      perm.push(usedchar);
    }

    findAllPermutations(arr);
    usedchar.pop();
    arr.splice(i,0,x);
  }
}

findAllPermutations([1,2,3]);

console.log(perm); //-> [[],[],[],[],[],[]]

I expect to get:

[[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

Can someone explain why I'm not getting this answer?

Answer 1

perm.push(usedchar);

needs to be

perm.push(usedchar.slice());

in order to clone the usedchar array, to snapshot it at that moment. Otherwise, you end up with six references to usedchar that had all kinds of stuff pushed in and then popped out, ending up empty at the end.

Answer 2

You're using the same array that holds your intermediate result as storage for your algorithm and things are getting mixed up. You need to either separate the two or make sure you clone your usedchar array before pushing it into your perm array. Something like this plunker: http://plnkr.co/edit/HQ6q8fo8S0K21cISQKVt

  var perm = [];
  var usedchar = [];
  var another = [];

  function findAllPermutations(arr) {


  for (var i = 0; i < arr.length; i++) {
    var x = arr.splice(i,1)[0];
    usedchar.push(x);

    if (usedchar.length == 3) {
      perm.push(usedchar.slice());
    };

    findAllPermutations(arr);
    usedchar.pop();
    arr.splice(i,0,x);
  };
};

findAllPermutations([1,2,3]);
console.log(perm);