二叉树深度搜索
1. 路径总和 II
前序操作和后序操作结合：
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<vector<int>> pathSum(TreeNode* root, int sum) {
vector<vector<int>> result;
vector<int> path;
int path_value = 0;
pathSum(root, path_value, sum, path, result);
return result;
}
void pathSum(TreeNode* node, int& path_value, int sum, vector<int>& path, vector<vector<int>>& result)
{
if( !node )
{
return ;
}
path_value += node->val; // 前序遍历的操作
path.push_back(node->val);
if( (!node->left) && (!node->right) && (path_value == sum) )
{
result.push_back(path);
}
pathSum(node->left, path_value, sum, path, result);
pathSum(node->right, path_value, sum, path, result);
path_value -= node->val; // 后序遍历的操作
path.pop_back();
}
};
2.二叉树的最近公共祖先
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
TreeNode* result = NULL;
vector<TreeNode*> path; // 临时路径
vector<TreeNode*> p_path; // 获取走向p的路径
vector<TreeNode*> q_path; // 获取走向q的路径
bool flag = false; // 标记是否找到需要查找到结点
pathWay(root, p, path, p_path, flag); // 获取p_path
path.clear(); // 清除path，为查找q的路径做准备
flag = false;
pathWay(root, q, path, q_path, flag); // 获取q_path
int len = (p_path.size() <= q_path.size()) ? p_path.size() : q_path.size();
for(int i = 0; i < len; ++i)
{
if( p_path[i] == q_path[i] )
{
result = p_path[i];
}
}
return result;
}
void pathWay(TreeNode* node, TreeNode* search_node, vector<TreeNode*>& path, vector<TreeNode*>& search_path , bool& flag)
{
if( !node || flag )
{
return ;
}
path.push_back(node);
if( search_node == node )
{
search_path = path;
flag = true;
}
pathWay(node->left, search_node, path, search_path, flag);
pathWay(node->right, search_node, path, search_path, flag);
path.pop_back();
}
};
3. 二叉树展开为链表
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
void flatten(TreeNode* root) {
TreeNode* last = NULL;
preorder(root, last);
return ;
}
void preorder(TreeNode* node, TreeNode*& last)
{
if( !node )
{
return ;
}
if( !node->left && !node->right ) // 如果为叶子节点
{
last = node;
return ;
}
TreeNode* left_node = node->left;
TreeNode* right_node = node->right;
TreeNode* left_last = NULL;
TreeNode* right_last = NULL;
if( left_node )
{
preorder(left_node, left_last);
node->left = NULL;
node->right = left_node;
last = left_last;
}
if( right_node )
{
preorder(right_node, right_last);
if( left_last )
{
left_last->right = right_node;
}
last = right_last;
}
}
};
二叉树层次遍历
4.二叉树的右视图
方法一：通过循环来记录层数
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<int> rightSideView(TreeNode* root) {
vector<int> result;
queue<TreeNode*> q;
if( root )
{
q.push(root);
}
while( !q.empty() )
{
int len = q.size();
for(int i=0; i<len; ++i)
{
TreeNode* node = q.front();
q.pop();
if( i == (len - 1))
{
result.push_back(node->val);
}
if( node->left )
{
q.push(node->left);
}
if( node->right )
{
q.push(node->right);
}
}
}
return result;
}
};
图的深度搜索/广度搜索
5.课程表
struct GraphNode // 图的邻接表数据结构
{
int label; // 图的顶点值
vector<GraphNode*> neighbors; // 相邻结点指针数组
GraphNode(int x) : label(x){}; // 构造函数
};
class Solution {
public:
bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
vector<GraphNode*> graph; // 申请一个有向图
vector<int> degree; // 每个顶点的度
for(int i=0; i<numCourses; ++i)
{
degree.push_back(0); // 初始化每个顶点的度
graph.push_back(new GraphNode(i)); // 初始化图的顶点
}
for(int i=0; i<prerequisites.size(); ++i) // 构造课程先后关系
{
GraphNode* begin = graph[prerequisites[i].second];
GraphNode* end = graph[prerequisites[i].first];
begin->neighbors.push_back(end);
degree[prerequisites[i].first]++;
}
queue<GraphNode*> q;
for(int i=0; i<numCourses; ++i) // 遍历图中度为0的顶点
{
if( degree[graph[i]->label] == 0 )
{
q.push(graph[i]);
}
}
while( !q.empty() ) // 通过queue来维护图中结点的度数
{
GraphNode* node = q.front();
q.pop();
for(int i=0; i<node->neighbors.size(); ++i)
{
degree[node->neighbors[i]->label]--;
if( degree[node->neighbors[i]->label] == 0 )
{
q.push(node->neighbors[i]);
}
}
}
for(int i=0; i<numCourses; ++i) // 删除图的顶点
{
delete graph[i];
}
for(int i=0; i<numCourses; ++i) // 遍历degree数组，如果有degree中有大于零的，即代表形成了环
{
if( degree[i] >0 )
{
return false;
}
}
return true;
}
};
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