LeetCode：Lowest Common Ancestor of a Binary Search Tree

问题描述：

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

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).”

        _______6______
       /              \
    ___2__          ___8__
   /      \        /      \
   0      _4       7       9
         /  \
         3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

思路：

递归调用思路较简单，如果两个节点都比根节点小，在左子树寻找，反之，如果两个节点都比根节点大，就在右子树找，如果都不满足，则一个在左子树，一个在右子树，返回根节点。

JAVA代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root == null || p == null || q == null){
            return null;
        }
        if(p == root || q == root){
            return root;
        }
        if(Math.max(p.val,q.val) < root.val){
            return lowestCommonAncestor(root.left,p,q);
        }
        else if(Math.min(p.val,q.val) > root.val){
            return lowestCommonAncestor(root.right,p,q);
         }
        else return root;
}
}