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  • LeetCode OJ 235. Lowest Common Ancestor of a Binary Search Tree

    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.

    由于这是一颗二叉搜索树,我们根据nodes v and w值的大小,能确定他们位于根节点的左边还是右边。如果两个节点都在根节点的左子树上,那么他们的LCA也肯定在根节点的左子树;如果两个节点都在根节点的右子树上,那么他们的LCA也肯定在根节点的右子树;如果两个节点分别位于根结的左右子树,那么根节点就是他们的LCA。否则的话直接返回根节点即可。代码如下:

     1 /**
     2  * Definition for a binary tree node.
     3  * public class TreeNode {
     4  *     int val;
     5  *     TreeNode left;
     6  *     TreeNode right;
     7  *     TreeNode(int x) { val = x; }
     8  * }
     9  */
    10 public class Solution {
    11     public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    12         if(root==null || p==null || q==null) return null;
    13         
    14         if(p.val<root.val && q.val<root.val){       //在根节点左边
    15             return lowestCommonAncestor(root.left, p, q);
    16         }
    17         else if(p.val>root.val && q.val>root.val){  //在根节点右边
    18             return lowestCommonAncestor(root.right, p, q);
    19         }
    20         else return root;                           //在根节点两侧,或者其中一个节点与根节点相等
    21     }
    22 }
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  • 原文地址:https://www.cnblogs.com/liujinhong/p/5476944.html
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