Published May 31, 2022
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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).”
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.
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.
Input: root = [1,2], p = 1, q = 2
Output: 1
[2, 10^5].-10^9 <= Node.val <= 10^9Node.val are unique.p != qp and q will exist in the tree./**
* Definition for a binary tree node.
* class TreeNode {
* val: number
* left: TreeNode | null
* right: TreeNode | null
* constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
* this.val = (val===undefined ? 0 : val)
* this.left = (left===undefined ? null : left)
* this.right = (right===undefined ? null : right)
* }
* }
*/
function lowestCommonAncestor(root: TreeNode | null, p: TreeNode | null, q: TreeNode | null): TreeNode | null {
const dfs = (node: TreeNode): TreeNode => {
if(node === null) {
return null;
}
// descendant of itself
if(node === p || node === q){
return node;
}
const left: TreeNode = dfs(node.left);
const right: TreeNode = dfs(node.right);
// split
if(left && right){
return node;
}
// can't find
if(!left && !right){
return null
}
return left ? left : right;
}
return dfs(root)
};