对称二叉树

递归写法

function help(root) {
    return isSymmetric(root, root);
}
  
function isSymmetric(node1, node2) {
    //判断两个节点都是否为空
    if (!node1 && !node2) {
      return true;
    }
    //判断两个节点不相等
    if (!node1 || !node2 || node1.val != node2.val) {
      return false;
    }

    return isSymmetric(node1.left, node2.right) && isSymmetric(node1.right, node2.left);
}

非递归写法

function isSymmetric(root) {
    if(!root) return true
    const queue = [root.left, root.right]
    let node1, node2

    while(queue.length) {
        node1 = queue.shift()
        node2 = queue.shift()

        if (!node1 && !node2) continue
        if (!node1 || !node2 || node1.val !== node2.val) return false
        
        //入队顺序，按照对称比对的顺序
        queue.push(node1.left)
        queue.push(node2.right)
        queue.push(node1.right)
        queue.push(node2.left)
    }

    return true
}