Question

link

Given a Binary Tree and a key, find distance of the closest leaf.

Examples:

          1
        /    \    
       2       3
             /   \  
            5     6   
           /       \
          7         8
         / \       /
        9  10     11

Closest key to '8' is '11', so distance is 1 for '8'
Closest key to '3' is '2', so distance is 2 for '3'
Closest key to '5' is either '9' or '10', so distance is 2 for '5'
Closest key to '2' is '2' itself, so distance is 0 for '2' 

Solution

traverse the given tree in preorder and keep track of ancestors (in a caching data struture, either it’s List or an array with a correct pointer)

When we find our target, we do 2 things:

1. find closest distance on lower subtrees of current node.

2. for every ancester, find the closest distance on lower subtrees, then add with distance to ancester.

Finally, return the smallest value seen above.

Code

Inspired by the code from G4G

int answer;

public int findClosest(TreeNode root, int key) {
    answer = Integer.MAX_VALUE;
    helper(root, key, new ArrayList<TreeNode>());
    return answer;
}

private void helper(TreeNode node, int key, List<TreeNode> path) {
    if (node == null) {
        return;
    } else if (node.val != key) {
        path.add(node);
        helper(node.left, key, path);
        helper(node.right, key, path);
        path.remove(path.size() - 1);
    } else {
        // key matches with current node value
        answer = lenToLowerLeaf(node);
        // answer initially = cloest leaf from lower

        int len = path.size();
        for (int i = 0; i < len; i++) {
            // for every ancestor, calculate distance and compare
            int ithToLowerLeaf = lenToLowerLeaf(path.get(i));
            answer = Math.min(answer, (len - i) + ithToLowerLeaf);
        }
    }
}

private int lenToLowerLeaf(TreeNode node) {
    if (node == null) {
        return Integer.MAX_VALUE;
    } else if (node.left == null && node.right == null) {
        return 0;
    } else {
        return 1 + Math.min(lenToLowerLeaf(node.left), lenToLowerLeaf(node.right));
    }
}