LeetCode-236.Lowest Common Ancestor of a Binary Tree

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.

可查找两个结点，并记录各自的路径，随时间复杂度也是O(n)，但较复杂

下面解法，时间复杂度O(n)

 1 public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {//树 递归 mytip
 2         if (null==root||root.val==p.val||root.val==q.val){
 3             return root;
 4         }
 5         TreeNode left = lowestCommonAncestor(root.left,p,q);
 6         TreeNode right = lowestCommonAncestor(root.right,p,q);
 7         if(null!=left&&null!=right){
 8             return root;
 9         }
10         else if (null ==right){
11             return left;
12         }
13         else{
14             return right;
15         }
16     }