相同的树

输入:       1         1
          / \       / \
         2   3     2   3

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

输出: true

输入:      1          1
          /           \
         2             2

        [1,2],     [1,null,2]

输出: false

输入:       1         1
          / \       / \
         2   1     1   2

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

输出: false

const isSameTree = function(p, q) {
    if (p === null && q === null) 
        return true
    else if (p === null || q === null || p.val !== q.val)
        return false
    else 
        return isSameTree(p.left, q.left) && isSameTree(p.right, q.right)
};