Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

判斷兩棵二叉樹樹是不是完全相同的
遞歸，先看兩個(gè)同一個(gè)位置的節(jié)點(diǎn)是不是相等（包括是否為空，值）：
如果是且不為空，就把這兩個(gè)節(jié)點(diǎn)左右節(jié)點(diǎn)分別傳進(jìn)去繼續(xù)往下比，看這個(gè)節(jié)點(diǎn)的左子樹和右子樹是不是相等，返回結(jié)果的與；
如果都為空返回true；
其他情況返回false；

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} p
 * @param {TreeNode} q
 * @return {boolean}
 */
var isSameTree = function(p, q) {
    if (p!==null&&q!==null) {
        if (p.val===q.val){
            return isSameTree(p.left,q.left) && isSameTree(p.right,q.right);
        } else {
            return false;
        }
    } else if (p===null&&q===null){
        return true;
    } else {
        return false;
    }
};