12. 对称二叉树

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
*/
var isSymmetric = function(root) {
  let stack = [root];
  while (stack.length) {
    let temp = [];
    for (let item of stack) {
      if (item) {
        temp.push(item.left);
        temp.push(item.right);
      }
    }
    if (stack.length > 1) {
      let start = 0, end = stack.length - 1;
      while (start < end) {
        if (stack[start]?.val !== stack[end]?.val) {
          return false;
        }
        ++start;
        --end;
      }
    }
    stack = temp;
  }
  return true;
};