recursive很容易写binary tree
iterative后来再补上:DFS vs BFS是最好的queue vs stack 实例
1, Depth first traversals:
inorder (left, root, right)
preorder (root, left, right)
postorder (left, right, root)
The root postion decides the order.
For example:
Inorder:
void helper(TreeNode* root, vector<int>&res)
{
if(root == NULL) return;
helper(root->left, res);
res.push_back(root->val);
helper(root->right, res);
}
Preorder:
void helper(TreeNode* root, vector<int>&res)
{
if(root == NULL) return;
res.push_back(root->val);
helper(root->left, res);
helper(root->right, res);
}
PostOrder:
void helper(TreeNode* root, vector<int>&res)
{
if(root == NULL) return;
helper(root->left, res);
helper(root->right, res);
res.push_back(root->val);
}
2, Breadth first traversal:
level order:
/**
* Definition for a binary tree node.
* 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>row;
int size = q.size();
for(int i = 0; i < size; i++)
{
TreeNode* cur = q.front();
q.pop();
row.push_back(cur->val);
if(cur->left) q.push(cur->left);
if(cur->right) q.push(cur->right);
}
res.push_back(row);
}
return res;
}
};