算法系列—— Lowest Common Ancestor of a Binary Search Tree
2017-08-23 17:09
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题目描述
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.
解题思路
对于二叉搜索树,公共祖先的值一定大于等于较小的节点,小于等于较大的节点。换言之,在遍历树的时候,如果当前结点大于两个节点,则结果在当前结点的左子树里,如果当前结点小于两个节点,则结果在当前节点的右子树里。程序实现
/** * 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||p==null||q==null) return null; if(root.val > p.val && root.val > q.val) return lowestCommonAncestor(root.left, p, q); if(root.val < p.val && root.val < q.val) return lowestCommonAncestor(root.right, p, q); return root; } }
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