给定一棵二叉树，任务是找到在左视图中可见的节点的总和。二叉树的左视图是从左侧查看树时可见的节点集。
例子：
Input:
1
/ \
2 3
/ \ \
4 5 6
Output: 7
1 + 2 + 4 = 7
Input:
1
/ \
2 3
\
4
\
5
\
6
Output: 18方法：这个问题可以使用简单的递归遍历来解决。我们可以通过向所有递归调用传递参数来跟踪节点的级别。这个想法是跟踪最大级别并以在右子树之前访问左子树的方式遍历树。每当遇到级别超过迄今为止最大级别的节点时，将节点的值添加到总和中，因为它是其级别中的第一个节点（注意左子树在右子树之前遍历）。
下面是上述方法的实现：
C++
// C++ implementation of the approach
#include
using namespace std;
class Node {
public:
int data;
Node *left, *right;
};
// A utility function to create
// a new Binary Tree Node
Node* newNode(int item)
{
Node* temp = new Node();
temp->data = item;
temp->left = temp->right = NULL;
return temp;
}
// Recursive function to find the sum of nodes
// of the left view of the given binary tree
void sumLeftViewUtil(Node* root, int level, int* max_level, int* sum)
{
// Base Case
if (root == NULL)
return;
// If this is the first Node of its level
if (*max_level < level) {
*sum += root->data;
*max_level = level;
}
// Recur for left and right subtrees
sumLeftViewUtil(root->left, level + 1, max_level, sum);
sumLeftViewUtil(root->right, level + 1, max_level, sum);
}
// A wrapper over sumLeftViewUtil()
void sumLeftView(Node* root)
{
int max_level = 0;
int sum = 0;
sumLeftViewUtil(root, 1, &max_level, &sum);
cout << sum;
}
// Driver code
int main()
{
Node* root = newNode(12);
root->left = newNode(10);
root->right = newNode(30);
root->right->left = newNode(25);
root->right->right = newNode(40);
sumLeftView(root);
return 0;
} Java
// Java implementation of the approach
// Class for a node of the tree
class Node {
int data;
Node left, right;
public Node(int item)
{
data = item;
left = right = null;
}
}
class BinaryTree {
Node root;
static int max_level = 0;
static int sum = 0;
// Recursive function to find the sum of nodes
// of the left view of the given binary tree
void sumLeftViewUtil(Node node, int level)
{
// Base Case
if (node == null)
return;
// If this is the first node of its level
if (max_level < level) {
sum += node.data;
max_level = level;
}
// Recur for left and right subtrees
sumLeftViewUtil(node.left, level + 1);
sumLeftViewUtil(node.right, level + 1);
}
// A wrapper over sumLeftViewUtil()
void sumLeftView()
{
sumLeftViewUtil(root, 1);
System.out.print(sum);
}
// Driver code
public static void main(String args[])
{
BinaryTree tree = new BinaryTree();
tree.root = new Node(12);
tree.root.left = new Node(10);
tree.root.right = new Node(30);
tree.root.right.left = new Node(25);
tree.root.right.right = new Node(40);
tree.sumLeftView();
}
}Python3
# Python3 implementation of the approach
# A binary tree node
class Node:
# Constructor to create a new node
def __init__(self, data):
self.data = data
self.left = None
self.right = None
# Recursive function to find the sum of nodes
# of the left view of the given binary tree
def sumLeftViewUtil(root, level, max_level, sum):
# Base Case
if root is None:
return
# If this is the first node of its level
if (max_level[0] < level):
sum[0]+= root.data
max_level[0] = level
# Recur for left and right subtree
sumLeftViewUtil(root.left, level + 1, max_level, sum)
sumLeftViewUtil(root.right, level + 1, max_level, sum)
# A wrapper over sumLeftViewUtil()
def sumLeftView(root):
max_level = [0]
sum =[0]
sumLeftViewUtil(root, 1, max_level, sum)
print(sum[0])
# Driver code
root = Node(12)
root.left = Node(10)
root.right = Node(20)
root.right.left = Node(25)
root.right.right = Node(40)
sumLeftView(root)C#
// C# implementation of the approach
using System;
// Class for a node of the tree
public class Node {
public int data;
public Node left, right;
public Node(int item)
{
data = item;
left = right = null;
}
}
public class BinaryTree {
public Node root;
public static int max_level = 0;
public static int sum = 0;
// Recursive function to find the sum of nodes
// of the left view of the given binary tree
public virtual void leftViewUtil(Node node, int level)
{
// Base Case
if (node == null) {
return;
}
// If this is the first node of its level
if (max_level < level) {
sum += node.data;
max_level = level;
}
// Recur for left and right subtrees
leftViewUtil(node.left, level + 1);
leftViewUtil(node.right, level + 1);
}
// A wrapper over leftViewUtil()
public virtual void leftView()
{
leftViewUtil(root, 1);
Console.Write(sum);
}
// Driver code
public static void Main(string[] args)
{
BinaryTree tree = new BinaryTree();
tree.root = new Node(12);
tree.root.left = new Node(10);
tree.root.right = new Node(30);
tree.root.right.left = new Node(25);
tree.root.right.right = new Node(40);
tree.leftView();
}
}Javascript
输出：
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