二叉树的前序、中序、后续遍历，递归、非递归实现

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前序遍历递归：

#include <vector>#include <iostream>using namespace std;struct TreeNode {int val;TreeNode *left;TreeNode *right;TreeNode(int x) : val(x), left(NULL), right(NULL) {}};void pre_order_recursive(TreeNode* root, vector<int>& result) {if(!root) return;result.push_back(root->val);if(root->left)pre_order_recursive(root->left, result);if (root->right)pre_order_recursive(root->right, result);}/*   1  /  \ 2    3/ \  / \       4  5 6   7*/int main(){TreeNode* n1 = new TreeNode(1);TreeNode* n2 = new TreeNode(2);TreeNode* n3 = new TreeNode(3);TreeNode* n4 = new TreeNode(4);TreeNode* n5 = new TreeNode(5);TreeNode* n6 = new TreeNode(6);TreeNode* n7 = new TreeNode(7);n1->left = n2; n1->right = n3;n2->left = n4; n2->right = n5;n3->left = n6; n3->right = n7;vector<int> res;pre_order_recursive(n1, res);for(int i = 0; i < res.size(); i++){cout << res[i] << " ";}system("PAUSE");}

中序遍历递归

void in_order_recursive(TreeNode* root, vector<int>& result) {if(root->left)in_order_recursive(root->left, result);if(root)result.push_back(root->val);if(root->right)in_order_recursive(root->right, result);}

后续遍历递归

void post_order_recursive(TreeNode* root, vector<int>& result) {if(root->left)post_order_recursive(root->left, result);if(root->right)post_order_recursive(root->right, result);if(root)result.push_back(root->val);}

前序遍历非递归

void pre_order(TreeNode* root, vector<int>& result) {stack<TreeNode*> st;while(root || !st.empty()){while (root){st.push(root);result.push_back(root->val);root = root->left;}if(!st.empty()){root = st.top();st.pop();root = root->right;}}}

中序遍历非递归

void in_order(TreeNode* root, vector<int>& result) {stack<TreeNode*> st;while(root || !st.empty()){while (root){st.push(root);root = root->left;}if(!st.empty()){root = st.top();result.push_back(root->val);st.pop();root = root->right;}}}

后续遍历非递归（增加指针指向上个遍历的节点，注意栈的push顺序与前序、中序不同）

void post_order(TreeNode* root, vector<int>& result) {stack<TreeNode*> st;TreeNode* pre = NULL;st.push(root);while(!st.empty()){root = st.top();if((root->left == NULL && root->right == NULL) || (pre == root->right) || (root->right == NULL && pre == root->left)){result.push_back(root->val);cout << root->val << " ";pre = root;st.pop();}else{if(root->right)st.push(root->right);if(root->left)st.push(root->left);}}}

（转）另一种方式（每个节点增加访问次数标识，push顺序与之前一致）

void postOrder2(BinTree *root)    //非递归后序遍历{    stack<BTNode*> s;    BinTree *p=root;    BTNode *temp;    while(p!=NULL||!s.empty())    {        while(p!=NULL)              //沿左子树一直往下搜索，直至出现没有左子树的结点         {            BTNode *btn=(BTNode *)malloc(sizeof(BTNode));            btn->btnode=p;            btn->isFirst=true;            s.push(btn);            p=p->lchild;        }        if(!s.empty())        {            temp=s.top();            s.pop();            if(temp->isFirst==true)     //表示是第一次出现在栈顶              {                temp->isFirst=false;                s.push(temp);                p=temp->btnode->rchild;                }            else                        //第二次出现在栈顶              {                cout<<temp->btnode->data<<" ";                p=NULL;            }        }    }    }

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