LC.P2583[二叉树中的第K大层和]

方法一：BFS+排序

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/**
 * 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 long kthLargestLevelSum(TreeNode root, int k) {
        Deque<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);
        List<Long> list = new ArrayList<>();
        while (!queue.isEmpty()) {
            long sum = 0;
            for (int size = queue.size(); size > 0; --size) {
                TreeNode node = queue.poll();
                sum += node.val;
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            list.add(sum);
        }
        int n = list.size();
        if (n < k) return -1;
        Collections.sort(list);
        return list.get(n - k);
    }
}
  • 时间复杂度：$O(nlogn)$
  • 空间复杂度：$O(n)$