二叉树的四种非递归遍历方式（前序、中序、后序、层次遍历）C++实现

最新推荐文章于 2023-04-01 14:22:01 发布

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本文详细介绍了二叉树的四种主要遍历算法：前序遍历、中序遍历、逆中序遍历及后序遍历的非递归实现方法。每种遍历算法都提供了具体的C++代码实现，帮助读者深入理解二叉树遍历的原理和应用。

【前序遍历】

void preOrderTraveralWithStack(TreeNode *root){	
	//NLR 根左右的顺序访问节点
        stack<TreeNode*> s;
        TreeNode *treeNode = root;
        while(treeNode!=NULL || !s.empty()){
            //迭代访问节点的左孩子，并入栈
            while(treeNode != NULL){
                s.push(treeNode);
                cout<<treeNode->val<<endl;
                treeNode = treeNode->left;
            }
            //若没有左孩子，则弹出栈顶节点，访问节点右孩子
            if(!s.empty()){
                treeNode = s.top();
                s.pop();
                treeNode = treeNode->right;
            }
        }
    }

【中序遍历的非递归方法】

void inOrderTraveralWithStack(TreeNode* root) {
     //RNL 右根左 中序遍历
     stack<TreeNode*> s;
     TreeNode* treeNode = root;
     while(treeNode!=NULL || !s.empty()){
         while(treeNode != NULL){
             s.push(treeNode);
             treeNode = treeNode->left;
         }
         if(!s.empty()){
             treeNode = s.top();
             s.pop();
             cout<<treeNode->val<<endl;//访问节点
             treeNode = treeNode->right;
         }
     }
 }

【逆中序遍历的非递归方法】

class Solution {
public:
   int cnt=0;
    int inOrderTraveralWithStack(TreeNode* root) {
        //RNL 右根左 中序遍历
        stack<TreeNode*> s;
        TreeNode* treeNode = root;
        while(treeNode!=NULL || !s.empty()){
            while(treeNode != NULL){
                s.push(treeNode);
                treeNode = treeNode->right;
            }
            if(!s.empty()){
                treeNode = s.top();
                s.pop();
                cout<<treeNode->val<<endl;//访问节点
                treeNode = treeNode->left;
            }
        }
        return 0;
    }
};

【后序遍历】

void postOrderTraveralWithStack(TreeNode* root){
    stack<TreeNode*> s;
    TreeNode *treeNode = root;
    TreeNode *lastVisit = NULL;   //标记每次遍历最后一次访问的节点
    while(treeNode || !s.empty()){//节点不为空，结点入栈，并且指向下一个左孩子
        while(treeNode){
            s.push(treeNode);
            treeNode = treeNode->left;
        }
        if(!s.empty()){
            treeNode = s.pop();
            if(treeNode->right == NULL || treeNode->right == lastVisit) {
                cout<<treeNode->val<<endl;
                lastVisit = treeNode;
                treeNode  = NULL;
            }else{
                s.push(treeNode);
                treeNode = treeNode->right;
            }
        }
    }
}

【后序遍历的非递归写法二】

void postOrderTraveralWithStack(TreeNode* root){
	stack<TreeNode*> s;
	TreeNode *treeNode = root;
	TreeNode *lastVisit = NULL; 
	while(treeNode||!s.empty()){
		while(treeNode){
			s.push(treeNode);
			p=p->left;
		}
		if(!s.empty()){
			treeNode=s.top();
			if(treeNode->right&&treeNode!=lastVisit){//若右边子
			//树存在，且从未被访问过
				treeNode=treeNode->right;//转向右子树
				s.push(treeNode);//将右子树根节点压入栈
				treeNode=treeNode->left;
			}else{
				s.pop();
				cout<<treeNode->val<<endl;
				lastVisit=treeNode;
				treeNode=NULL;
			}
		}
	}
}

【层次遍历】

void levelOrderWithQueue(TreeNode *root){
	Queue<TreeNode*> q;
	q.push(root);
	TreeNode* treeNode;
	while(!q.empty()){
		treeNode=q.front();
		q.pop();
		cout<<treeNode->val<<endl;
		if(treeNode->left)
			s.push(treeNode->left);
		if(treeNode->right)
			s.push(treeNode->right);
	}
}