DevExCode

2641. Cousins in Binary Tree II

Hash TableTreeDepth-First SearchBreadth-First SearchBinary Tree

Explanation:

  • We will perform a level-order traversal of the binary tree.
  • While traversing each level, we will keep track of the nodes that are cousins and calculate their sum.
  • We will update the node values with the sum of their cousins' values.
  • To identify cousins, we will keep track of the parent node of each node.
  • We will use a queue to perform the level-order traversal.

Time Complexity: O(N) where N is the number of nodes in the tree.
Space Complexity: O(N) for the queue.

:

View Leetcode Problem
class Solution {
    public TreeNode replaceCousins(TreeNode root) {
        if (root == null) {
            return null;
        }
        
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            Map<TreeNode, Integer> parentMap = new HashMap<>();
            Map<Integer, Integer> cousinsSumMap = new HashMap<>();
            
            for (int i = 0; i < size; i++) {
                TreeNode current = queue.poll();
                TreeNode parent = parentMap.getOrDefault(current, null);
                
                if (parent != null && parentMap.get(parent) != parentMap.get(current)) {
                    cousinsSumMap.put(parentMap.get(current), cousinsSumMap.getOrDefault(parentMap.get(current), 0) + current.val);
                    cousinsSumMap.put(parentMap.get(parent), cousinsSumMap.getOrDefault(parentMap.get(parent), 0) + current.val);
                }
                
                if (current.left != null) {
                    queue.offer(current.left);
                    parentMap.put(current.left, current);
                }
                
                if (current.right != null) {
                    queue.offer(current.right);
                    parentMap.put(current.right, current);
                }
            }
            
            for (Map.Entry<Integer, Integer> entry : cousinsSumMap.entrySet()) {
                for (Map.Entry<TreeNode, Integer> nodeEntry : parentMap.entrySet()) {
                    if (nodeEntry.getValue() == entry.getKey()) {
                        nodeEntry.getKey().val = entry.getValue();
                    }
                }
            }
        }
        
        return root;
    }
}

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