1123. Lowest Common Ancestor of Deepest Leaves

Given the root of a binary tree, return the lowest common ancestor of its deepest leaves.

Recall that:

  • The node of a binary tree is a leaf if and only if it has no children
  • The depth of the root of the tree is 0. if the depth of a node is d, the depth of each of its children is d + 1.
  • The lowest common ancestor of a set S of nodes, is the node A with the largest depth such that every node in S is in the subtree with root A.

Note: This question is the same as 865: https://leetcode.com/problems/smallest-subtree-with-all-the-deepest-nodes/

 

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4]
Output: [2,7,4]
Explanation: We return the node with value 2, colored in yellow in the diagram.
The nodes coloured in blue are the deepest leaf-nodes of the tree.
Note that nodes 6, 0, and 8 are also leaf nodes, but the depth of them is 2, but the depth of nodes 7 and 4 is 3.

Example 2:

Input: root = [1]
Output: [1]
Explanation: The root is the deepest node in the tree, and it's the lca of itself.

Example 3:

Input: root = [0,1,3,null,2]
Output: [2]
Explanation: The deepest leaf node in the tree is 2, the lca of one node is itself.

 

Constraints:

  • The number of nodes in the tree will be in the range [1, 1000].
  • 0 <= Node.val <= 1000
  • The values of the nodes in the tree are unique.

Rust Solution

struct Solution;
use rustgym_util::*;

trait Postorder {
    fn postorder(&self) -> (usize, TreeLink);
}

impl Postorder for TreeLink {
    fn postorder(&self) -> (usize, TreeLink) {
        use std::cmp::Ordering::*;
        if let Some(node) = self {
            let left = &node.borrow().left;
            let right = &node.borrow().right;
            let (height_l, left_lca) = left.postorder();
            let (height_r, right_lca) = right.postorder();
            match height_l.cmp(&height_r) {
                Less => (height_r + 1, right_lca),
                Greater => (height_l + 1, left_lca),
                Equal => (height_l + 1, self.clone()),
            }
        } else {
            (0, None)
        }
    }
}

impl Solution {
    fn lca_deepest_leaves(root: TreeLink) -> TreeLink {
        let (_, lca) = root.postorder();
        lca
    }
}

#[test]
fn test() {
    let root = tree!(1, tree!(2), tree!(3));
    let res = tree!(1, tree!(2), tree!(3));
    assert_eq!(Solution::lca_deepest_leaves(root), res);
    let root = tree!(1, tree!(2, tree!(4), None), tree!(3));
    let res = tree!(4);
    assert_eq!(Solution::lca_deepest_leaves(root), res);
    let root = tree!(1, tree!(2, tree!(4), tree!(5)), tree!(3));
    let res = tree!(2, tree!(4), tree!(5));
    assert_eq!(Solution::lca_deepest_leaves(root), res);
    let root = tree!(
        1,
        tree!(2, tree!(3, None, tree!(6)), tree!(4, None, tree!(5))),
        None
    );
    let res = tree!(2, tree!(3, None, tree!(6)), tree!(4, None, tree!(5)));
    assert_eq!(Solution::lca_deepest_leaves(root), res);
}

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