Binary Tree Level Order Traversal II

最新推荐文章于 2025-08-22 19:21:44 发布

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本文介绍了一种从叶子节点到根节点的二叉树层序遍历算法实现，通过递归方式获取每个层级的节点值，并将其按从下到上的顺序返回。

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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],
]

confused what "{1,#,2,3}" means?

/**

* Definition for binary tree
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int getDepth(TreeNode * node){
if (!node) return 0;
else{
return max(getDepth(node->left),
getDepth(node->right)) + 1;
}
}

void preorderTraverse(TreeNode * node, int depth,
vector<vector<int> > & solution){
if (!node)
return;
else{
solution[depth].push_back(node->val);
preorderTraverse(node->left, depth-1, solution);
preorderTraverse(node->right, depth-1, solution);
}
}

vector<vector<int> > levelOrderBottom(TreeNode *root) {
int treeDepth = getDepth(root);
vector<int> temp;
vector<vector<int> > solution(treeDepth, temp);

preorderTraverse(root, treeDepth-1, solution);

return solution;
}
};