二叉树的遍历（递归/非递归）

前序

class Solution {
public:
    std::vector vec;
    vector preorderTraversal(TreeNode* root) {
        if(nullptr==root) return vec;
        vec.push_back(root->val);
       preorderTraversal(root->left);
       preorderTraversal(root->right);
       return vec;
    }

    
};

vector preorderTraversal(TreeNode* root) {
      std::vector res;
      if(nullptr==root) return res;

      std::stack st;
      st.push(root);
      while(!st.empty())
      {
        TreeNode* p = st.top();
        res.push_back(p->val);
        st.pop();
        if(p->right) st.push(p->right);
        if(p->left) st.push(p->left);
      }
      return res;
}

中序

class Solution {
public:

    void inOrderTraverse(std::vector& res, TreeNode* node){
        if(nullptr==node) return;
        inOrderTraverse(res,node->left);
        res.push_back(node->val);
        inOrderTraverse(res,node->right);
    }

    vector inorderTraversal(TreeNode* root) {
        std::vector res;
        inOrderTraverse(res,root);
        return res;
    }

   
};

    vector inorderTraversal(TreeNode* root) {
        std::vector res;
        std::stack st;
        TreeNode* cur = root;

        while(cur || !st.empty())
        {
            if(cur){
                st.push(cur);
                cur=cur->left;
            }else{
                TreeNode* p = st.top();
                res.push_back(p->val);
                st.pop();
                cur = p->right;
            }
        }

        return res;
    }

后序

class Solution {
public:

    void postOrderTraverse(std::vector& res, TreeNode* node){
        if(nullptr==node) return;
        postOrderTraverse(res,node->left);
        postOrderTraverse(res,node->right);
        res.push_back(node->val);

    }

    vector postorderTraversal(TreeNode* root) {
      std::vector res;
      postOrderTraverse(res,root);
      return res;
    }
};

    vector postorderTraversal(TreeNode* root) {
      std::vector res;
      if(root==nullptr) return res;
      std::stack st;
      st.push(root);

      while(!st.empty())
      {
        TreeNode* p = st.top();
        st.pop();
        res.push_back(p->val);
        if(p->left) st.push(p->left);
        if(p->right) st.push(p->right);
      }

      std::reverse(res.begin(),res.end());
      return res;
    }