Javascript：使用递归就地展平多维数组

Question

我有以下代码可以展平多维数组

var x = [[[2, 3], 4], 5, [6, 7]];

function flatten(arr) {

  for (var i = 0; i < arr.length; i++) {
    if (arr[i].constructor === Array) {
      subArr = arr[i];
      // Shift the array down based on the space needed for the sub array
      for (var j = arr.length + subArr.length - 2; j > i + subArr.length - 1; j--) {
        arr[j] = arr[j - subArr.length + 1];
      }
      // Insert sub array elements where they belong
      for (var k = 0; k < subArr.length; k++) {
        arr[i + k] = subArr[k]; 
      }
      // Look at arr[i] again in case the first element in the subarray was another array;
      i--;
    }
  }
}

flatten(x);

这里是 JSBin：http://jsbin.com/gazagemabe/edit?js,console

我想用递归来做这个，但我最终卡住了。我试图在不保留临时数组的情况下做到这一点，但事情似乎变得不正常了。我觉得我缺少递归的一些核心原则。

我意识到我需要一个基本案例。我能想到的唯一情况是当前处于展平状态的数组没有子数组。但是因为我总是通过引用传递数组，所以我没有什么可以 return.

我的伪代码是

function flatten(arr) 

  loop through arr
    if arr[index] is an array
      increase the array length arr[index] length
      flatten(arr[index])
    else
      // unsure what to do here to modify the original array

Answer 1

这是一个完全就地工作的纯递归版本，不构建中间数组。

// Deep-flatten an array starting at index `i`.
function flatten(a, i = 0) {                                                                              

  if (i >= a.length)                                                                                      
    // Null case--we are off the end of the array.                                                        
    return;                                                                                               

  var elt = a[i];                                                                                         

  if (!Array.isArray(elt))                                                                                
    // Element is not an array. Proceed to next element.                                                  
    i++;                                                                                                  

  else                                                                                                    
    // Element is an array.                                                                               
    // Unroll non-empty arrays; delete empty arrays.                                                      

    if (elt.length) {                                                                                     
      // Non-empty array.                                                                                 
      // Unroll it into head and tail.                                                                    

      // Shift elements starting at `i` towards end of array.
      // Have to do this recursively too--no loops!                                             
      (function shift(a, i, n = a.length) {                                                               
        if (n > i ) a[n] = a[n-1], shift(a, i, --n);                                                      
      }(a, i));                                                                                           

      // Replace elt with its head, and place tail in slot we opened up.                                  
      a[i] = elt.shift();                                                                                 
      a[i + 1] = elt;                                                                                     
    }                                                                                                     

  else                                                                                                    
    // Array is empty.                                                                                    
    // Delete the element and move remaining ones toward beginning of array. 
    // Again, have to do this recursively!                             
    (function unshift(a, i) {                                                                             
      if (i < a.length) a[i] = a[i+1], unshift(a, ++i);                                                   
      else a.length--;                                                                                    
    }(a, i));                                                                                             

  flatten(a, i);                                                                                          

}                                                                                                         

var arr = [[[2, 3], 4], 5, [6, 7]];                                                                   
flatten(arr);                                                                                             
console.log(arr);                                                                                         

[2, 3, 4, 5, 6, 7]

这个解决方案使用了ES6的点点滴滴，比如默认函数参数和Array.isArray。如果您没有可用的 ES6，请相应地进行调整。

Answer 2

从内到外进行递归展平。首先对您找到的数组调用 flatten 函数，然后将该数组（现在是扁平的）的内容移动到父数组中。通过数组向后循环，这样您就不必为插入的项目调整循环变量。

如您所知，您插入的数组是平面的，您可以使用 splice 方法用其项目替换数组。

它是这样工作的：

start with
[[[2, 3], 4], 5, [6, 7]]

flatten [6,7] (which is already flat) and insert:
[[[2, 3], 4], 5, 6, 7]

flatten [[2, 3], 4] recursively calls flatten [2,3] and inserts in that array:
[[2, 3, 4], 5, 6, 7]
then it inserts [2, 3, 4]:
[2, 3, 4, 5, 6, 7]

代码：

function flatten(arr) {
  for (var i = arr.length - 1; i >= 0; i--) {
    if (arr[i].constructor === Array) {
      flatten(arr[i]);
      Array.prototype.splice.apply(arr, [i, 1].concat(arr[i]));
    }
  }
}

var x = [[[2, 3], 4], 5, [6, 7]];

flatten(x);

// Show result in snippet
document.write(JSON.stringify(x));

Javascript：使用递归就地展平多维数组

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