1650 Lowest Common Ancestor of a Binary Tree III
Given two nodes of a binary tree p and q, return their lowest common ancestor (LCA).
Each node will have a reference to its parent node. The definition for Node is below:
According to the definition of LCA on Wikipedia: "The lowest common ancestor of two nodes p and q in a tree T is the lowest node 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:
/*
// Definition for a Node.
class Node {
public int val;
public Node left;
public Node right;
public Node parent;
};
*/
class Solution {
public Node lowestCommonAncestor(Node p, Node q) {
// base case
if (p == q){
return p;
}
if (p.parent == q.parent){
return p.parent;
}
Node runner1 = p;
Node runner2 = q;
while(runner1 != runner2){
if (runner1 == null){
runner1 = q;
}else{
runner1 = runner1.parent;
}
if (runner2 == null){
runner2 = p;
}else{
runner2 = runner2.parent;
}
}
return runner1;
}
}
// TC: O(n)
// SC: O(1)
/*
r1. = p
r2 = q
3
/ \
5 (p) 1
/ \ / \
6 2 0 8
/\
7 4(q)
3
/ \
5(r1) 1
/ \ / \
6 2 0 8
/\
7 4(r2)
q p
4 - 2 - 5 3
r1 r1 =q
r2
1 ------o--- 1 + 2: ------o--- --o---
2 --o--- 2 + 1: --o-- -------o---
*/