LeetCode——Lowest Common Ancestor of a Binary Search Tree

Description：

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.

给出一颗二叉查找树，两个节点，找出这两个节点的最近公共祖先。

思路：递归可以使这道题变得异常的简单。如果两个节点是在根节点两侧，那么最近的公共祖先就是这个根节点，如果是左侧（右侧），则把根节点递归到左子树（右子树）。

/**
 * 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;
        }
        int max = p.val > q.val ? p.val : q.val;
        int min = p.val < q.val ? p.val : q.val;
        if(max < root.val) {
            return lowestCommonAncestor(root.left, p, q);
        }
        else if(min > root.val) {
            return lowestCommonAncestor(root.right, p, q);
        }
        else {
            return root;
        }
        
    }
}