剑指offer 25 二叉树中和为某一值的路径

非递归方法:

class Solution {
public:
    vector<vector<int>> FindPath(TreeNode* root,int expectNumber) {
        vector<vector<int>> result;
        if(!root)
            return result;
        stack<TreeNode *> s;
        s.push(root);
        TreeNode *cur=root;
        TreeNode *last=NULL;
        vector<int> curpath;
        int sum=0;
        while(!s.empty())
        {
            if(cur==NULL)
            {
                if((s.top())->right&&(s.top())->right!=last)
                {
                    cur=(s.top())->right;
                }
                else
                {
                    last=s.top();
                    s.pop();
                    sum-=last->val;
                    curpath.pop_back();
                }
            }
            else
            {
                s.push(cur);
                curpath.push_back(cur->val);
                sum+=cur->val;
                if(!cur->left&&!cur->right&&sum==expectNumber)
                {
                    result.push_back(curpath);
                }
                cur=cur->left;
            }
                
        }
        return result;    
    }
};

要注意转向右子树时的情况！！

对容器vector，push_back()是从容器末尾插入元素，pop_back()是从容器末尾删除元素。stack没有迭代器，只能插入和得到栈顶元素。

递归方法如下：

class Solution {
public:
    vector<vector<int>> FindPath(TreeNode* root,int expectNumber) {
        vector<vector<int>> result;
        if(!root)
            return result;
        int cursum=0;
        vector<int> path;
        FindPath(root,expectNumber, result, path, cursum);
        return result;
    }
    void FindPath(TreeNode* root, int expectNumber, vector<vector<int>> &result, vector<int> &path, int cursum)
    {
        cursum+=root->val;
        path.push_back(root->val);
        
        if(!root->left&&!root->right&&cursum==expectNumber)
        {
            result.push_back(path);
        }
        if(root->left!=NULL)
            FindPath(root->left,expectNumber, result, path, cursum);
        if(root->right!=NULL)
            FindPath(root->right,expectNumber, result, path, cursum);
        path.pop_back();
    }
};