二叉树的遍历：前序、中序、后序和层次遍历

层次遍历

/**
 * 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;
        queue<TreeNode *> q;
        if(root==NULL) return res ;
        q.push(root);
        vector<int> tmp;
        TreeNode * p = NULL;
        while(!q.empty()){
            int n = q.size();
            tmp.clear();
            for(int i=0;i<n;i++){
                p = q.front();
                q.pop();
                tmp.push_back(p->val);
                if(p->left) q.push(p->left);
                if(p->right) q.push(p->right);
            } 
            res.push_back(tmp);
        }
        return res;
    }
};

参考资料：https://blog.youkuaiyun.com/zgaoq/article/details/79089819
前序
非递归实现

/**
 * 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> res;
        stack<TreeNode *> s;
        TreeNode * p = root;
        while(!s.empty() || p){
            while(p){
                res.push_back(p->val);
                s.push(p);
                p=p->left;
            }
            p = s.top();
            s.pop();
            p=p->right;
        }
        return res;
    }
};

中序
非递归实现

/**
 * 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> inorderTraversal(TreeNode* root) {
        vector<int> res;
        TreeNode * p = root;
        stack<TreeNode *> s;
        while(!s.empty() || p){
            while(p){
                s.push(p);
                p = p->left;
            }
            p = s.top();
            s.pop();
            res.push_back(p->val);
            p = p->right;
        }
        return res;
    }
};

后序
非递归实现

/**
 * 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> postorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode *> s;
        TreeNode * p = root, * preVisit = NULL;
        while(p){
              s.push(p);
              p = p->left;
        }
        while(!s.empty()){           
            p = s.top();
            if(p->right==NULL || p->right==preVisit){
                s.pop();
                res.push_back(p->val);
                preVisit = p;
            }else{
                p = p->right;
                while(p){
                    s.push(p);
                    p=p->left;
                }
            }
        }
        return res;
    }
};