617. Merge Two Binary Trees

A full binary tree is a binary tree where each node has exactly 0 or 2 children.

Return a list of all possible full binary trees with N nodes.  
Each element of the answer is the root node of one possible tree.

Each node of each tree in the answer must have node.val = 0.

You may return the final list of trees in any order.

example 1
Input: 7
Output: [[0,0,0,null,null,0,0,null,null,0,0],
[0,0,0,null,null,0,0,0,0],[0,0,0,0,0,0,0],
[0,0,0,0,0,null,null,null,null,0,0],[0,0,0,0,0,null,null,0,0]]

根据输入节点的个数求所有可能的满二叉树的情况。
左右子树的节点个数都是奇数。左子树节点个数为奇数i，右子树节点个数为N-1-i。
循环所有的奇数节点，用递归构造左右子树。每次递归返回一个数组，是所有两二叉树情况的节点，左右节点分别存在两个数组中。遍历所有的左右节点，插入到二叉树的左右子树。

var allPossibleFBT = function(N) {
       var result = []
       if(N==1){
           var leafnode = new TreeNode(0)
           result.push(leafnode)
           return result
       }
       if(N%2==0 || N<1) {
           return result
       }
 
       for(var i=1; i<N ;i+=2){
           var leftlist = allPossibleFBT(i)
           var rightlist = allPossibleFBT(N-i-1)
           for(var j=0; j<leftlist.length; j++){
               for(var k=0; k<rightlist.length; k++){
                   var node = new TreeNode(0)
                   node.left = leftlist[j]
                   node.right = rightlist[k]
                   result.push(node)
               }
           }
       }
       return result
   };