[LeetCode]236. Lowest Common Ancestor of a Binary Tree

https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/

找出两个节点的公共祖先，树是普通二叉树

lowestCommonAncestor找在root为根节点的子树上遍历到的第一个p或者q，如果两个都遍历到就返回root，如果p和q都不在这个子树上就返回null，否则返回p和q里面先遍历到的那个。因此如果left和right都不是null，那么就返回root

public class Solution {    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {        if (root == null || root == p || root == q) {            return root;        }        TreeNode left = lowestCommonAncestor(root.left, p, q);        TreeNode right = lowestCommonAncestor(root.right, p, q);        return left == null ? right : right == null ? left : root;    }}