2.PostOrder - Using Stack.txt

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    public:
    void postorder(TreeNode* root,vector<int> &ans)
    {
        if(root==NULL)
        {
            return;
        }
        else
        {
            postorder(root->left,ans);
            postorder(root->right,ans);
            ans.push_back(root->val);
        }
    }
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ans;
        postorder(root,ans);
        return ans;
    }
};




class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ans;
        if(root==NULL)
        {
            return ans;
        }
        else if(root->left==NULL&root->right==NULL)
        {
            ans.push_back(root->val);
            return ans;
        }
        else
        {
            stack<TreeNode*> st;
            TreeNode* curr = root;
            TreeNode* temp;
            while(curr!=NULL||!st.empty())
            {
                if(curr!=NULL)
                {
                    st.push(curr);
                    curr=curr->left;
                }
                else
                {
                    temp = st.top()->right;
                    if(temp==NULL)
                    {
                        temp = st.top();
                        st.pop();
                        ans.push_back(temp->val);
                        while(!st.empty()&&temp==st.top()->right)
                        {
                            temp = st.top();
                            st.pop();
                            ans.push_back(temp->val);
                        }
                    }
                    else
                    {
                        curr = temp;
                    }
                }
            }
            return ans;
        }
    }
};