leetcode之Binary Tree Level Order Traversal II

最新推荐文章于 2019-06-07 14:12:14 发布

原创最新推荐文章于 2019-06-07 14:12:14 发布 · 686 阅读

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本文介绍了一种二叉树的层次遍历算法，并实现了遍历结果的逆序输出。通过广度优先搜索（BFS）遍历二叉树的所有节点，按层收集节点值，最后反转整个结果列表实现从叶节点到根节点的层次遍历。

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题目大意:

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]
]

意思就是:

类似leetcode之Binary Tree Level Order Traversal, 只是需要对输出结果进行逆序

代码如下:

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        	if(!root){
				return result;
			}
			bfsTravel(root);
			reverse(result.begin(), result.end());
			return result;
    }
    void bfsTravel(TreeNode* node){
			queue<qInsert> Q;
			qInsert first(0, node);
			Q.push(first);
			vector<int> temp;
			int last_level = 1;
			while(!Q.empty()){
				qInsert current = Q.front();
				Q.pop();
				//temp.push_back(current.val->val);
				if(last_level != current.level){
					vector<int> mm;
					mm.push_back(current.val->val);
					result.push_back(mm);
				}
				else{
					result[current.level].push_back(current.val->val);
				}
				if(current.val->left){
					qInsert second(current.level+1, current.val->left);
					Q.push(second);

				}
				if(current.val->right){
					qInsert third(current.level+1, current.val->right);
					Q.push(third);
				}
				last_level = current.level;
			}
		}
	private:
		vector< vector<int> > result;
		struct qInsert{
			TreeNode* val;
			int level;
			qInsert(int x, TreeNode* y) : level(x), val(y){}
		};
};