算法

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

一开始没仔细看题目是搜索树，写的是普通二叉树的解法：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    TreeNode result = null;
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        findNode(root,p,q);
        return result;
    }
    
    
    public int findNode(TreeNode root, TreeNode p, TreeNode q)
    {
        if(root==null || result !=null) return 0;
        int lv = findNode(root.left,p,q);
        int rv = findNode(root.right,p,q);
        if(result!=null) return 0;
        if(root==p)
        {
            if(lv+rv==2) result = p;
            return 1;
        }
        if(root==q)
        {
            if(lv+rv==1) result = q;
            return 2;
        }
        if(lv + rv ==3) result = root;
        return lv+rv;
    }
}

搜索树就简单很多了：

/**
 * 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(p.val < q.val)
        {
            if(root.val > q.val) return lowestCommonAncestor(root.left,p,q);
            else if(root.val < p.val) return lowestCommonAncestor(root.right,p,q);
            else return root;
        }
        else return lowestCommonAncestor(root,q,p);
    }
}