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LeetCode-Binary Tree Preorder Traversal

使用了三种方法对二叉树进行前序遍历，具体可以参考二叉树相关知识点总结。

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/**
 * 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> result;
        preorder3(root,result);
        return result;
    }
private:
    void preorder1(TreeNode* root, vector<int>& result){
        if(root==nullptr) return;
        result.push_back(root->val);
        preorder1(root->left,result);
        preorder1(root->right,result);
    }
    void preorder2(TreeNode* root, vector<int>& result){
        if(root==nullptr) return;
        stack<TreeNode*> s;
        TreeNode* n=root;
        while(!s.empty() || n!=nullptr){
            while(n!=nullptr){
                s.push(n);
                result.push_back(n->val);
                n=n->left;
            }
            if(!s.empty()){
                n=s.top();
                s.pop();
                n=n->right;
            }
        }
    }
    void preorder3(TreeNode* root, vector<int>& result){
        if(root==nullptr) return;
        stack<TreeNode*> s;
        s.push(root);
        TreeNode* n;
        while(!s.empty()){
            n=s.top();
            s.pop();
            result.push_back(n->val);
            if(n->right) s.push(n->right);
            if(n->left) s.push(n->left);
        }
    }
};