二叉树的前序遍历三种方法

</pre>第一种最常用的就是直接暴力递归的方法：<p></p><p></p><pre name="code" class="cpp">/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in vector which contains node values.
     */
    vector<int> preorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> result;
        if(root){
            trav_pre(root, result);
        }
        return result;
    }
private:
    void trav_pre(TreeNode *x, vector<int> &ret) {
        if (x == nullptr){
            return;
        }
        ret.push_back(x -> val);
        trav_pre(x -> left, ret);
        trav_pre(x -> right, ret);
    }
};
           

第二种是使用分而治之的方法：

这种方法在现实中比较常用，相当于本身前序遍历，就是为了实现一个接口，这样通过不断调用本接口来解决问题。

class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in vector which contains node values.
     */
    vector<int> preorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> result;
        if (root == nullptr) {
            return result;
        }
        //devide
        vector<int> left = preorderTraversal(root -> left);
        vector<int> right =  preorderTraversal(root -> right);
        //conquer
        result.push_back(root -> val);
        result.insert(result.end(), left.begin(), left.end());
        result.insert(result.end(), right.begin(), right.end());
        return result;
    }
};
           

第三种是使用非递归的方法：

这个方法的关键就是引入栈，用栈来代替递归的思想

class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in vector which contains node values.
     */
    vector<int> preorderTraversal(TreeNode *root) {
        // write your code here
    vector<int> result;
    stack<TreeNode *> sta;
    while(1){
       travel_pre(root, result, sta);
       if(sta.empty()){
           break;
       }
       root = sta.top();
       sta.pop();
    }
    return result;
    }
private:
     void travel_pre(TreeNode *x, vector<int> &ret, stack<TreeNode *> &st) {
        while (x){
            ret.push_back(x -> val);
            st.push(x -> right);
            x = x -> left;
        }
         return;
     }
};
           

不用递归的另外一种写法：

class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in vector which contains node values.
     */
    vector<int> preorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> result;
        stack<TreeNode *> sta;
        if (root == nullptr){
            return result;
        }
        
        sta.push(root);
        while(!sta.empty()) {
            TreeNode * tem = sta.top();
            result.push_back(tem -> val);
            sta.pop();
            if (tem -> right) {//因为是前序遍历，栈是先进后出，因此先放右边再放左边
                sta.push(tem -> right);
            }
            if (tem -> left) {
                sta.push(tem -> left);
            }
        }
        return result;
    }
};