各种排列组合

function getGroup(arr) {
    let r = [];
    function f(n){
        if(n==0){
            r.push([arr[0]]);
            return r;
        }
        f(n-1).forEach(v=>{
            v = v.slice();
            v.push(arr[n])
            r.push(v);
        });
        r.push([arr[n]]);
        return r;
    }
    return f(arr.length-1);
}

输入：[1,2,3]

输出：[ [ 1 ], [ 1, 2 ], [ 2 ], [ 1, 3 ], [ 1, 2, 3 ], [ 2, 3 ], [ 3 ] ]

function getGroupStr(str) {
    let r = [];
    function f(n){
        if(n==0){
            r.push(str[0]);
            return r;
        }
        f(n-1).forEach(v=>{
            r.push(v+str[n]);
        });
        r.push(str[n]);
        return r;
    }
    return f(str.length-1);
}

输入：“abc”

输出：['a', 'ab', 'b', 'ac', 'abc', 'bc', 'c']

// 返回分组的索引
function groupSplitIdx (arrSize, n) {
  var r = [];

  function _(t, i) {
    if (t.length === n) {
      r[r.length] = t.slice();
      return;
    }
    for (; i < arrSize; i++) {
      t.push(i);
      _(t, i + 1);
      t.pop();
    }
  }
  _([], 0);
  return r;
}

输入：5,3

输出：

[
  [ 0, 1, 2 ], [ 0, 1, 3 ],
  [ 0, 1, 4 ], [ 0, 2, 3 ],
  [ 0, 2, 4 ], [ 0, 3, 4 ],
  [ 1, 2, 3 ], [ 1, 2, 4 ],
  [ 1, 3, 4 ], [ 2, 3, 4 ]
]

function groupSplit (arr, size) {
  var r = [];

  let b = new Array(size);
  function _(start, end, x) {
    if (x == size) {
      r.push(b.slice());
      return;
    }
    if ((end - start) < (size - x)) {
      return;
    }
    b[x] = arr[start];
    _(start + 1, end, x + 1);
    _(start + 1, end, x);
  _(0, arr.length, 0);
  return r;
}

输入：[1,2,3,4,5], 3

输出：

[
  [ 1, 2, 3 ], [ 1, 2, 4 ],
  [ 1, 2, 5 ], [ 1, 3, 4 ],
  [ 1, 3, 5 ], [ 1, 4, 5 ],
  [ 2, 3, 4 ], [ 2, 3, 5 ],
  [ 2, 4, 5 ], [ 3, 4, 5 ]
]

function groupSplitIdx2 (m) {
  let r = [];
  function _(i) {
    if (i == 0) {
      r.push([0]);
      return r;
    }
    _(i - 1).forEach(v=>{
      v = v.slice();
      v.push(i);
      r.push(v);
    });
    r.push([i]);
    return r;
  }
  return _(m-1);
}

　输入：4

　输出：

[
  [ 0 ],       [ 0, 1 ],
  [ 1 ],       [ 0, 2 ],
  [ 0, 1, 2 ], [ 1, 2 ],
  [ 2 ],       [ 0, 3 ],
  [ 0, 1, 3 ], [ 1, 3 ],
  [ 0, 2, 3 ], [ 0, 1, 2, 3 ],
  [ 1, 2, 3 ], [ 2, 3 ],
  [ 3 ]
]

function groupSplitIdx3 (arrSize, n) {
  var r = [];

  function _(t, i) {
    if (t.length === n) {
      return;
    }
    for (; i < arrSize; i++) {
      t.push(i);
      r.push(t.slice());
      _(t, i + 1);
      t.pop();
    }
  }
  _([], 0);
  return r;
}

输入：5,3

输出：

[
  [ 0 ],       [ 0, 1 ],    [ 0, 1, 2 ],
  [ 0, 1, 3 ], [ 0, 1, 4 ], [ 0, 2 ],
  [ 0, 2, 3 ], [ 0, 2, 4 ], [ 0, 3 ],
  [ 0, 3, 4 ], [ 0, 4 ],    [ 1 ],
  [ 1, 2 ],    [ 1, 2, 3 ], [ 1, 2, 4 ],
  [ 1, 3 ],    [ 1, 3, 4 ], [ 1, 4 ],
  [ 2 ],       [ 2, 3 ],    [ 2, 3, 4 ],
  [ 2, 4 ],    [ 3 ],       [ 3, 4 ],
  [ 4 ]
]

 数组的排列组合

function arrCom(arr) {
  function _(n) {
    if (n === 1) {
      return arr[0].map(a=>[a]);
    } else {
      let r = [];
      _(n - 1).forEach(item=>{
        arr[n - 1].forEach(a=>{
          let item2 = item.slice();
          item2.push(a);
          r.push(item2);
        });
      });
      return r;
    }
  }
  return _(arr.length);
}

输入：

[

[1, 2], [3, 4], [5, 6, 7, 9, 10]

]

输出：

[
[ 1, 3, 5 ], [ 1, 3, 6 ],
[ 1, 3, 7 ], [ 1, 3, 9 ],
[ 1, 3, 10 ], [ 1, 4, 5 ],
[ 1, 4, 6 ], [ 1, 4, 7 ],
[ 1, 4, 9 ], [ 1, 4, 10 ],
[ 2, 3, 5 ], [ 2, 3, 6 ],
[ 2, 3, 7 ], [ 2, 3, 9 ],
[ 2, 3, 10 ], [ 2, 4, 5 ],
[ 2, 4, 6 ], [ 2, 4, 7 ],
[ 2, 4, 9 ], [ 2, 4, 10 ]
]

 快速组合的方式：

function arrCom(arr, cb) {
  let r = [];
  let lastIdx = arr.length - 1;
  function _(arr, n) {
    let a = arr[n];
    if (n === lastIdx) {
      if (a.length) {
        for (let i = 0, j = a.length; i < j; i++) {
          r.push(a[i]);
          cb(r);
          r.pop();
        }
      } else {
        cb(r);
      }
      return;
    }

    if (a.length) {
      for (let i = 0, j = a.length; i < j; i++) {
        r.push(a[i]);
        _(arr, n + 1);
        r.pop();
      }
    } else {
      _(arr, n + 1);
    }
  }

  _(arr, 0);
}