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[LeetCode] Lowest Common Ancestor of a Binary Search Tree

发布时间:2015-08-16 16:00:07
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 o ...

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.

     这道题不难。主要是先要搞清楚LCA的定义。

     然后LCA呢只会分为三种情况,如果P,Q中最大值的那个都小于root的值,那么LCA只能在左边了。反正如果P,Q中最小值的那个都大于root的值,那么LCA肯定只能在右边。如果这两种情况都不是,那么就是最特殊的一种。LCA直接就是root。因为没有办法把这两个node一起放在一边子树去了。

     代码如下。~

/** * Definition for a binary tree node. * public class TreeNode { *   int val; *   TreeNode left; *   TreeNode right; *   TreeNode(int x) { val = x; } * } */public class Solution {  public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {    if(root==null){      return null;    }    if(Math.max(p.val,q.val)<root.val){      return lowestCommonAncestor(root.left,p,q);    }else if(Math.min(p.val,q.val)>root.val){      return lowestCommonAncestor(root.right,p,q);    }    return root;  }}

原标题:[LeetCode] Lowest Common Ancestor of a Binary Search Tree

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