Leetcode-标签为Tree 235. Lowest Common Ancestor of a Binary Search Tree

原题

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.

题目分析

求两个节点的最低的祖先。最低祖先可能为两个节点中的一个。

代码实现

        public TreeNode LowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q)        {            if (root == null)                return null;            if (p == root || q == root) //等于root，则最小祖先一定为root                return root;            bool pOnLeft = isOn(root.left, p);            bool qOnLeft = isOn(root.left, q);            if (pOnLeft != qOnLeft)                return root;            if (pOnLeft == true && qOnLeft == true)                return LowestCommonAncestor(root.left, p, q);            return LowestCommonAncestor(root.right, p, q);        }        //位于node上吗        private bool isOn(TreeNode node, TreeNode goal)        {            if (goal == null)                return true;            if (node == null)                return false;            if (node == goal)                return true;            if (isOn(node.left, goal) == true || isOn(node.right, goal) == true)                return true;            return false;        }