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 p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”
Given the following binary tree: root = [3,5,1,6,2,0,8,null,null,7,4]
Example 1:
Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.
Example 2:
Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.
Note:All of the nodes’ values will be unique.p and q are different and both values will exist in the binary tree.
难度：medium
题目：给定一二叉树，找出给定的两个结点的最低层的共同祖先结点。
根据 LCA 维基百科定义，最低公共祖先定义为两个节点 p 和 q 之间的最低公共祖先，它是 T 中同时具有 p 和 q 作为子代的最低节点 (我们允许一个节点作为自身的子代)
思路：递归，后续遍历。
Runtime: 5 ms, faster than 100.00% of Java online submissions for Lowest Common Ancestor of a Binary Tree.Memory Usage: 33.9 MB, less than 69.83% of Java online submissions for Lowest Common Ancestor of a Binary Tree.
/**
* 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 || root == p || root == q) {
return root;
}
TreeNode left = lowestCommonAncestor(root.left, p, q);
TreeNode right = lowestCommonAncestor(root.right, p, q);
return left == null ? right : right == null ? left : root;
}
}