LeetCode 235 Lowest Common Ancestor of a Binary Search Tree

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.

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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。

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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;    }