题目描述

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.

分析

BST，性质是：根结点都比左结点大，比右结点小。所以给定两个结点，找公共祖先，就是找值在两个结点间的结点。

代码

    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {

        if (root == null || p == null || q == null) {
            return null;
        }

        int m = Math.min(p.val, q.val);
        int n = Math.max(p.val, q.val);

        while (root != null) {
            if (root.val < m) {
                root = root.right;
            } else if (root.val > n) {
                root = root.left;
            } else {
                return root;
            }
        }

        return null;
    }