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Binary Tree Level Order Traversal 二叉树层序遍历

Given a binary tree, return the level order traversal of its nodes‘ values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3   /   9  20    /     15   7

 

return its level order traversal as:

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

层序遍历二叉树是典型的广度优先搜索BFS的应用,但是这里稍微复杂一点的是,我们要把各个层的数分开,存到一个二维向量里面,大体思路还是基本相同的,建立一个queue,然后先把根节点放进去,这时候找根节点的左右两个子节点,这时候去掉根节点,此时queue里的元素就是下一层的所有节点,用一个for循环遍历它们,然后存到一个一维向量里,遍历完之后再把这个一维向量存到二维向量里,以此类推,可以完成层序遍历。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> > levelOrder(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.push_back(oneLevel);        }        return res;    }};

 

Binary Tree Level Order Traversal 二叉树层序遍历