78. Subsets

题目：
Given a set of distinct integers, nums, return all possible subsets.
Note: The solution set must not contain duplicate subsets.
For example,If nums = [1,2,3], a solution is:

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

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题目大意，就是找出一个数组中的所有子集。

解答：参照leetcode中的讨论

屏幕快照 2016-12-05 下午8.00.58.png

1代表会取到nums中的对应的值，0代表没有。

代码：

/** 
  * @param {number[]} nums 
  * @return {number[][]} 
*/
var subsets = function(nums) { 
    if (nums.length === 0) 
        return []; 
    var len = nums.length, 
        count = Math.pow(2, len), 
        result = []; 
    for (var i = 0; i < count; i++) {
       // to binary string 
      var str = i.toString(2); 
      // get one subset from binary string 
      var subset = []; 
    // iterate the string from back to front since our binary string is 
    // e.g.: "11" instead of "011" when i = 3  
    for (var k = str.length - 1; k >= 0; k--) {
       if (str[k] == '1') 
          subset.unshift(nums[len - str.length + k]); 
    } 
    result.push(subset); 
 } 
 return result;
};