# 对称二叉树-简单

    1
   / \
  2   2
 / \ / \
3  4 4  3

var isSymmetric = function (root) {
  if (root === null) return true;
  function compare(leftRoot, rightRoot) {
    if (leftRoot === null && rightRoot === null) return true;
    if (leftRoot === null || rightRoot === null) return false;
    if (leftRoot.val !== rightRoot.val) return false;
    return (
      compare(leftRoot.left, rightRoot.right) &&
      compare(leftRoot.right, rightRoot.left)
    );
  }
  return compare(root.left, root.right);
};

var isSymmetric = function (root) {
  if (!root) return true;
  let queue = [root];

  while (queue.length) {
    let size = queue.length;
    const levels = [];
    while (size-- > 0) {
      const ele = queue.shift();
      if (ele === null) {
        levels.push(ele);
      } else {
        levels.push(ele.val);
        queue.push(ele.left);
        queue.push(ele.right);
      }
    }
    let i = 0;
    j = levels.length - 1;
    while (i < j) {
      if (levels[i] !== levels[j]) {
        return false;
      }
      i++;
      j--;
    }
  }

  return true;
};