[LeetCode] BFS-DFS 236. Lowest Common Ancestor of a Binary Tree

문제

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

 

Constraints:

• The number of nodes in the tree is in the range [2, 105].
• -109 <= Node.val <= 109
• All Node.val are unique.
• p != q
• p and q will exist in the tree.

---

문제풀이

p = 5, q = 1라면 모든 그래프를 다 돌 필요는 없다 

 

이거 이외에는 다 돌 필요 있다.

 

p = 6, q = 4여도 다 돌아야한다. => 다 돈 후 left([5,6,2,null,null,7,4]), right(null)을 비교해서 아직 남아있는 5(root)를 정답으로 한다

---

코드

// DFS

var lowestCommonAncestor = function(root, p, q){
    if(!root) return null;
    if(root === p || root === q) return root;
    
    const left = lowestCommonAncestor(root.left, p, q);
    const right = lowestCommonAncestor(root.right, p, q);
    
    if(!left && !right) return null;
    if(left && right) return root;
    
    if(root !== null) return left || right;
}