LeetCode (236):Lowest Common Ancestor of a Binary Search Tree

標籤：

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.

解題思路:

節點a與節點b的公用祖先c一定滿足：a與b分別出現在c的左右子樹上（如果a或者b本身不是祖先的話）。

Java代碼：

class TreeNode {     int val;     TreeNode left;     TreeNode right;     TreeNode(int x) { val = x; } }public class Solution {     public static TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {         if(root==null) return null;         if(root==p||root==q) return root;         TreeNode L=lowestCommonAncestor(root.left,p,q);         TreeNode R=lowestCommonAncestor(root.right,p,q);         if(L!=null&&R!=null) return root;        return L!=null?L:R;        }//test    public static void main(String[] args) {        // TODO Auto-generated method stub        TreeNode tr=new TreeNode(1);        TreeNode nodeB=new TreeNode(2);        TreeNode nodeC=new TreeNode(3);        TreeNode nodeD=new TreeNode(4);        TreeNode nodeE=new TreeNode(5);        tr.left=nodeB;        tr.right=nodeC;        tr.left.right=nodeD;            tr.left.left=nodeE;            System.out.print(lowestCommonAncestor(tr, nodeC, nodeE).val);    }}

 

LeetCode (236):Lowest Common Ancestor of a Binary Search Tree