【done】剑指offer68：二叉树最近公共祖先

LCA（lowest common ancestor）问题
力扣，【二叉搜索树】https://leetcode.cn/problems/er-cha-sou-suo-shu-de-zui-jin-gong-gong-zu-xian-lcof/description/
【普通二叉树】https://leetcode.cn/problems/er-cha-shu-de-zui-jin-gong-gong-zu-xian-lcof/

1.二叉搜索树

思路：https://leetcode.cn/problems/er-cha-sou-suo-shu-de-zui-jin-gong-gong-zu-xian-lcof/solutions/216894/mian-shi-ti-68-i-er-cha-sou-suo-shu-de-zui-jin-g-7/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        while (root != null) {
            if (p.val < root.val && q.val < root.val) {
                root = root.left;
            } else if (p. val > root.val && q.val > root.val) {
                root = root.right;
            } else {
                break;
            }

        }
        return root;
    }
}

2.普通二叉树

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null) {
            return null;
        }
        if (root == p || root == q) {
            return root;
        }
        TreeNode leftNode = lowestCommonAncestor(root.left, p, q);
        TreeNode rightNode = lowestCommonAncestor(root.right, p, q);
        if (leftNode != null && rightNode != null) {
            return root;
        }
        if (leftNode == null && rightNode == null) {
            return null;
        }
        return leftNode == null ? rightNode : leftNode;
    }
}