Leetcode 107 Binary Tree Level Order Traversal II 层次遍历及其变形

题目分析

Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

层次遍历变形

显然这是一道典型的层次遍历问题，但是要求从最后一层开始遍历，直到第一层为止。对此的解决思路是首先由上而下进行遍历，再将遍历的结果翻转即可。

AC代码 (>98.67%)

class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<TreeNode> queue = new ArrayList<>();
        List<List<Integer>> result = new ArrayList<>();
        if (root==null) return result;
        queue.add(root);
        int direct = 0;
        while (!queue.isEmpty()){
            List<Integer> addi = new ArrayList<>();
            int num = queue.size();
            for (int i=0;i<num;i++){
                TreeNode node = queue.remove(0);
                addi.add(node.val);
                if (node.left!=null) queue.add(node.left);
                if (node.right!=null) queue.add(node.right);
            }
            result.add(addi);
        }
        Collections.reverse(result);
        return result;
    }
}