面試OR筆試31——二叉樹中序遍歷第k節點

1 題目及要求

1.1 題目描述

給一個二叉樹，請找出其中的中序遍歷的第k節點。

 

2 解答

2.1 題目分析

中序遍歷時每訪問一個節點後k減一，直到k==1時的節點就是所求節點。

 

2.2 代碼

#include <iostream>
#include <stack>
using namespace std;

struct TreeNode {
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

// 遞歸實現
TreeNode* kthTreeNode1(TreeNode *root, int &k){
	if(!root||k<1) return nullptr;
	TreeNode *np = nullptr;
	np = kthTreeNode1(root->left,k);
	if(!np){
		if(k==1) np = root;
		--k;
	}
	if(!np) np = kthTreeNode1(root->right,k);
	return np;
}

// 循環實現
TreeNode* kthTreeNode2(TreeNode *root, int k){
	stack<TreeNode*>st;
	while(!st.empty() || root){
		if(root){
			st.push(root);
			root = root->left;
		}
		else{
			root = st.top();
			st.pop();
			if(k==1) return root;
			--k;
			root = root->right;
		}
	}
	return nullptr;
}

int main(){
	TreeNode ht[]={1,2,3,4,5,6,7};
	ht[3].left = ht+1; ht[3].right = ht+5;
	ht[1].left = ht;   ht[1].right = ht+2;
	ht[5].left = ht+4; ht[5].right = ht+6;
	
	TreeNode *np =nullptr;
	for(int k1(1),k2;k1<8;++k1){
		k2 = k1;
		cout << "求第 " << k1 << " 個節點: ";
		np = kthTreeNode1(ht+3,k2);
		np?(cout << np->val):(cout << "NULL");cout << "    ";
		np = kthTreeNode2(ht+3,k1);
		np?(cout << np->val):(cout << "NULL"); cout << endl;
	}
    return 0;
}