leetcode : Binary Tree Level Order Traversal II

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本文介绍了一种从底部向上进行层序遍历二叉树的方法，并提供了一个迭代实现的C++代码示例。该算法从根节点开始，逐层访问每个节点，最终将结果倒序存储以达到从下到上的遍历效果。

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,#,#,15,7},

return its bottom-up level order traversal as:

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

从底部层序遍历其实还是从顶部开始遍历，只不过最后存储的方式有所改变。代码如下：

// Iterative

class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        vector<vector<int> > res;
        if (root == NULL) return res;

        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            vector<int> oneLevel;
            int size = q.size();
            for (int i = 0; i < size; ++i) {
                TreeNode *node = q.front();
                q.pop();
                oneLevel.push_back(node->val);
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
            }
            res.insert(res.begin(), oneLevel);
        }
        return res;
    }
};