【LeetCode-144】Binary Tree Preorder Traversal(C++)
2016-09-17 15:33
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题目要求:完成二叉树的前序遍历。
解题方法:
1.递归的方法。很简单。
/**
* 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<int> preorderTraversal(TreeNode* root) {
vector<int> vl;
if(root==NULL)
return vl;
vector<int> vr;
vl=preorderTraversal(root->left);
vr=preorderTraversal(root->right);
vl.insert(vl.begin(),root->val);
for(int i=0;i<vr.size();i++){
vl.push_back(vr[i]);
}
return vl;
}
};2.题目中有这样一句话,Note: Recursive
solution is trivial, could you do it iteratively? 递归的方式很简单,你能用循环的方法做吗?所以再用循环的方法做一遍。因为前序遍历存在回溯的过程,所以需要用到栈来存储遍历过的节点。
/**
* 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<int> preorderTraversal(TreeNode* root) {
vector<int> v;
if(root==NULL)
return v;
stack<TreeNode*> s;
TreeNode *p=root;
while(p){
s.push(p);
v.push_back(p->val);
p=p->left;
}
while(!s.empty()){
p=s.top();
s.pop();
if(p->right){
p=p->right;
while(p){
s.push(p);
v.push_back(p->val);
p=p->left;
}
}
}
return v;
}
};
解题方法:
1.递归的方法。很简单。
/**
* 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<int> preorderTraversal(TreeNode* root) {
vector<int> vl;
if(root==NULL)
return vl;
vector<int> vr;
vl=preorderTraversal(root->left);
vr=preorderTraversal(root->right);
vl.insert(vl.begin(),root->val);
for(int i=0;i<vr.size();i++){
vl.push_back(vr[i]);
}
return vl;
}
};2.题目中有这样一句话,Note: Recursive
solution is trivial, could you do it iteratively? 递归的方式很简单,你能用循环的方法做吗?所以再用循环的方法做一遍。因为前序遍历存在回溯的过程,所以需要用到栈来存储遍历过的节点。
/**
* 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<int> preorderTraversal(TreeNode* root) {
vector<int> v;
if(root==NULL)
return v;
stack<TreeNode*> s;
TreeNode *p=root;
while(p){
s.push(p);
v.push_back(p->val);
p=p->left;
}
while(!s.empty()){
p=s.top();
s.pop();
if(p->right){
p=p->right;
while(p){
s.push(p);
v.push_back(p->val);
p=p->left;
}
}
}
return v;
}
};
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