二叉树的迭代遍历

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {    // 前序遍历
        stack<TreeNode*> st;
        if (root) {
            st.push(root);
        }
        vector<int> result;
        while(st.empty()==0){
            TreeNode *n=st.top();
            result.push_back(n->val);
            st.pop();
            if(n->right) st.push(n->right);
            if(n->left)  st.push(n->left);
        }
        return result;
    }
};

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {    // 中序遍历
        vector<int> result;
        stack<TreeNode*> st;
        if(root==NULL) return {};
        st.push(root);
        while(!st.empty()){
            TreeNode* n=st.top();
            st.pop();
            if(n->right){
                st.push(n->right);
                n->right=NULL;
            }
            if(n->left==NULL){
                result.push_back(n->val);
            }
            else{
                st.push(n);
                st.push(n->left);
                n->left=NULL;
            }
        }
        return result;
    }
};

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {    // 后序遍历
        vector<int> result;
        stack<TreeNode*> st;
        if(root==NULL) return {};
        st.push(root);
        while(!st.empty()){
            TreeNode* n=st.top();
            st.pop();
            if(n->left==NULL&&n->right==NULL){
                result.push_back(n->val);
            }
            else{
                st.push(n);
                if(n->right){
                    st.push(n->right);
                    n->right=NULL;
                } 
                if(n->left){
                    st.push(n->left);
                    n->left=NULL;
                }

            }
        }
        return result;
    }
};