437. Path Sum III

root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8

10
/  \
5   -3
/ \    \
3   2   11
/ \   \
3  -2   1

Return 3\. The paths that sum to 8 are:

1\.  5 -> 3
2\.  5 -> 2 -> 1
3\. -3 -> 11

class Solution {
public:
int pathSum(TreeNode* root, int sum) {
int ans=0;
dfs2(root,sum,ans);
return ans;
}
void dfs(TreeNode* root, int sum,int mysum,int &ans){
if(!root)return;
mysum+=root->val;
if(mysum==sum)++ans;
dfs(root->left,sum,mysum,ans);
dfs(root->right,sum,mysum,ans);
}
void dfs2(TreeNode* root,int sum,int &ans){
if(!root)return;
dfs(root,sum,0,ans);
dfs2(root->left,sum,ans);
dfs2(root->right,sum,ans);
}
};

class Solution {
public:
int pathSum(TreeNode* root, int sum) {
int ans=0;
dfs(root,sum,0,ans,0);
return ans;
}
void dfs(TreeNode* root, int sum,int mysum,int &ans,bool flag){
if(!root)return;
mysum+=root->val;
if(mysum==sum){++ans;}
dfs(root->left,sum,mysum,ans,flag);
dfs(root->right,sum,mysum,ans,flag);
if(flag)return;
dfs(root->left,sum,0,ans,1);
dfs(root->right,sum,0,ans,1);
}
};