572. Subtree of Another Tree

• Runtime: 155 ms, faster than 13.96%
• Memory Usage: 45.8 MB, less than 46.49%
• 时间复杂度O(n)，空间复杂度O(n)

/**
 * 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
 * @param {TreeNode} subRoot
 * @return {boolean}
 */
var isSubtree = function(root, subRoot) {
  if (!root) return false;
  if (isSameTree(root, subRoot)) return true;
  return isSubtree(root.left, subRoot) || isSubtree(root.right, subRoot);
};

var isSameTree = function(a, b) {
  if (!a && !b) return true;
  if (!a || !b) return false;
  return a.val === b.val && isSameTree(a.left, b.left) && isSameTree(a.right, b.right);
}