【LeetCode】236.Lowest Common Ancestor of a Binary Tree(Medium)解题报告

【LeetCode】236.Lowest Common Ancestor of a Binary Tree(Medium)解题报告

题目地址：https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/description/

题目描述：

  Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

  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).”

_______3______
/              \
___5__          ___1__
/      \        /      \
6      _2       0       8
/  \
7   4

  For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

  求二叉树的最近公共祖先

Solution:

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
采用后序遍历
time:O(n)
space:O(n)
*/
class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if(root == null || root == p || root == q) return root;

TreeNode left = lowestCommonAncestor(root.left,p,q);
TreeNode right = lowestCommonAncestor(root.right,p,q);

if(left != null && right != null){
return root;
}
return left == null ? right : left;
}
}

Date：2018年3月11日