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LeetCode 235.Lowest Common Ancestor of a Binary Search Tree

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Question

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).” 

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     _______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.

Explanation

既然是二叉搜索树，那就可以根据值来进行判断。分治递归。

Code

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