Lowest Common Ancestor of a Binary Tree

题目地址：
https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/#/description

题目：

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        _______3______
       /              \
    ___5__          ___1__
   /      \        /      \
   6      _2       0       8
         /  \
         7   4

For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

解题思路：
这道题主要就是recursion，当找到其中某个节点的时候就返回该节点。 follow up:
如果是bst的话，只需要比较root节点的值和给定的p和q的值得大小，思路同样是recursion。

代码：

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

follow up：

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    if(root.val > p.val && root.val > q.val){
        return lowestCommonAncestor(root.left, p, q);
    }
    else if(root.val < p.val && root.val < q.val){
        return lowestCommonAncestor(root.right, p, q);
    }
    return root;
}