PattySung

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).”

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.

Example 3:

Input: root = [1,2], p = 1, q = 2
Output: 1

<解題>

如果(root == null || root == p || root == q) return root，直接是他自己本身
建立左、右邊的尋找方式


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);
        if(left != null && right != null)   return root; //表示 p 和 q 分別在左子樹和右子樹中
        return left != null ? left : right; //表示 p 和 q 同時存在於左子樹或右子樹中，或者其中一個節點是另一個節點的祖先。
    }
}

Time: O(N) Space: O(H)

此題可和Leetcode-235. Lowest Common Ancestor of a Binary Search Tree做比較

Leetcode-236. Lowest Common Ancestor of a Binary Tree

<解題>