236. Lowest Common Ancestor of a Binary Tree

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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.

Solution

這題很巧妙的使用lowestCommonAncestor function的特性

當 Tree 不包含 p, q時，回傳會是null

當Tree只含其中之一，那回傳會是p or q

當Tree包含p, q, 那回傳的就是會是lowestCommonAncestor

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
       if(root == null || root = q || root == p) return root;
       TreeNode left = lowestCommonAncestor(root.left, p ,q);
       TreeNode right = lowestCommonAncestor(root.right, p ,q);
       if(left != null && left != null){
          return root;
       }
       return left == null ? right : left;
    }
}