Leetcode 235 Lowest Common Ancestor of a Binary Search Tree

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本文介绍了一种寻找二叉搜索树中两个指定节点的最低公共祖先的方法，并提供了具体的实现代码。通过递归和迭代的方式，算法有效地定位了目标节点路径并找到了最低公共祖先。

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.

public class Solution235 {

    public boolean findPath(TreeNode root, TreeNode p, List<TreeNode> result) {
        if(root == null) return false;

        if(root == p) {
            result.add(root);
            return true;
        }

        if(root.val >= p.val) {
            result.add(root.left);
            root = root.left;
        } else {
            result.add(root.right);
            root = root.right;
        }

        findPath(root,p,result);
        return true;
    }

    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        List<TreeNode> listp = new ArrayList<TreeNode>();
        List<TreeNode> listq = new ArrayList<TreeNode>();

        listp.add(root);
        listq.add(root);

        findPath(root, p, listp);
        findPath(root, q, listq);

        TreeNode ans = root;

        for (int i = 1, j = 1; i < listp.size() && j < listq.size(); i++, j++) {
            if(listp.get(i).equals(listq.get(j))) {
                ans = listp.get(i);
            } else {
                break;
            }
        }

        return ans;
    }
    public static void main(String[] args) {

        TreeNode p = null, q = null;

        TreeNode root = new TreeNode(6);
        root.left = new TreeNode(2);
        p = root.left;
        root.right = new TreeNode(8);
        //q = root.right;
        root.left.left = new TreeNode(0);
        root.left.right = new TreeNode(4);
        q = root.left.right;
        root.left.right.left = new TreeNode(3);
        root.left.right.right = new TreeNode(5);
        root.right.left = new TreeNode(7);
        root.right.right = new TreeNode(9);

        TreeNode ans = new Solution235().lowestCommonAncestor(root, p, q);

        System.out.println(ans.val);
    }
}