Lowest Common Ancestor of a Binary Search Tree

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.

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思路：树的问题多递归，由于是二叉搜索树，左孩子小于根节点，右孩子大于根节点，所以可以根据2个节点和根节点之间的关系来确定谁是最小公共祖先。

代码如下:

/** * 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(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;        }    }}