Find Largest Value in Each Tree Row

ID: 515; medium

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Solution 1 (Java)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Queue<TreeNode> q = new ArrayDeque<>();
        q.offer(root);
        
        while (!q.isEmpty()) {
            int n = q.size();
            int max = Integer.MIN_VALUE;
            for (int i = 0; i < n; i++) {
                TreeNode curr = q.poll();
                max = Math.max(max, curr.val);
                if (curr.left != null)
                    q.offer(curr.left);
                if (curr.right != null)
                    q.offer(curr.right);
            }
            res.add(max);
        }
        
        return res;
    }
}

Notes

• BFS

Solution 2 (Java)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        largestValuesHelper(root, 0, res);
        return res;
    }
    
    private void largestValuesHelper(TreeNode root, int level, List<Integer> res) {
        if (root == null) return;
        if (level == res.size()) {
            res.add(root.val);
        } else {
            res.set(level, Math.max(res.get(level), root.val));
        }
        largestValuesHelper(root.left, level + 1, res);
        largestValuesHelper(root.right, level + 1, res);
    }
}

Notes

• DFS