Binary Tree Level Order Traversal

OJ: https://oj.leetcode.com/problems/binary-tree-level-order-traversal/

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]
]
思想: 若递归,传入层号。若迭代,使用队列,在每层结束时,加入一个标记。
方法一:递归:
/**
* Definition for binary tree
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
void levelPath(TreeNode* root, int level, vector<vector<int> > &path) {
if(root == NULL) return;
level < path.size() ? path[level].push_back(root->val) : path.push_back(vector<int>(1, root->val));
levelPath(root->left, level+1, path);
levelPath(root->right, level+1, path);
}
class Solution {
public:
vector<vector<int> > levelOrder(TreeNode *root) {
vector<vector<int> > path;
levelPath(root, 0, path);
return path;
}
};

方法二:迭代

/**
* 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> > vec;
if(root == NULL) return vec;
queue<TreeNode*> qu;
qu.push(root);
qu.push(0);
vector<int> vec2;
while(!qu.empty()) {
TreeNode *p = qu.front();
qu.pop();
if(!p) {
if(vec2.size()) { vec.push_back(vec2); vec2.clear();}
if(!qu.empty()) qu.push(0);
}else {
vec2.push_back(p->val);
if(p->left) qu.push(p->left);
if(p->right) qu.push(p->right);
}
}
return vec;
}
};

Binary Tree Level Order Traversal II

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},

    3
/ \
9 20
/ \
15 7

return its bottom-up level order traversal as:

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

思想: 目前用两种方法:1 同上,最后将结果反转一下。 2.先求出最大层数,再层序遍历。(也许还有更好的方法)
1.
void levelPath(TreeNode* root, int level, vector<vector<int> > &path) {
if(root == NULL) return;
level < path.size() ? path[level].push_back(root->val) : path.push_back(vector<int>(1, root->val));
levelPath(root->left, level+1, path);
levelPath(root->right, level+1, path);
}
class Solution {
public:
vector<vector<int> > levelOrderBottom(TreeNode *root) {
vector<vector<int> > path;
levelPath(root, 0, path);
return vector<vector<int> > (path.rbegin(), path.rend());
}
};

2.

/**
* Definition for binary tree
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
int getLevel(TreeNode *root) {
if(root == NULL) return -1;
return max(getLevel(root->left), getLevel(root->right)) + 1;
}
void getLevel2(TreeNode *root, int curL, vector<vector<int> > &vec) {
if(root == NULL) return;
vec[curL].push_back(root->val);
getLevelBottom(root->left, curL-1, vec);
getLevelBottom(root->right,curL-1, vec);
}
class Solution {
public:
vector<vector<int> > levelOrderBottom(TreeNode *root) {
int L = getLevel(root);
vector<vector<int> > vec(L+1, vector<int>());
getLevelBottom(root, L, vec);
return vec;
}
};

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