Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.
According to the : “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
Pseudocode
-dfs to traverse BT, bfs not suitable here
- set up conditions to return LCA node
Solution
var lowestCommonAncestor = function (root, p, q) {
function walk(node) {
// base condition
if (!node) {
return null;
}
if (node === p || node === q) {
return node;
}
// pre
// recurse
const walkLeft = walk(node.left);
const walkRight = walk(node.right);
// post
if (walkLeft && walkRight) {
return node;
} else {
return walkLeft || walkRight;
}
}
return walk(root);
};
