层次遍历构造二叉树

/**
 * Definition for a binary tree node.
 */
function TreeNode(val){
     this.val = val;
     this.left = this.right = null;
}

function createBinaryTree(arr){
    var p = new TreeNode(arr[0]);
    var k = 0;
    var len = arr.length;
    var buildTree = function(node, i){
       if(2*i+1<len){
            node.left = new TreeNode(arr[2*i+1]);
            node.left && buildTree(node.left, 2*i+1)
        }
        if(2*i+2<len){
            node.right = new TreeNode(arr[2*i+2]);
            node.right && buildTree(node.right, 2*i+2);
        }
    };
    buildTree(p, 0);
    return p;
}

var arr = [3,9,20,null,null,15,7];
var tree = createBinaryTree(arr);
console.log(tree);