Leetcode oj java Path Sum III

一、问题描述：

You are given a binary tree in which each node contains an integer value.

Find the number of paths that sum to a given value.

The path does not need to start or end at the root or a leaf, but it must go downwards (traveling only from parent nodes to child nodes).

The tree has no more than 1,000 nodes and the values are in the range -1,000,000 to 1,000,000.

Example:

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

二、解决思路：

先序遍历所有的节点，递归计算路径中和为sum的值。需要注意的特殊情况是  a-b-c  是一条符合要求的路径， a-b-c-d-e (d+e=0) 也是一条符合要求的路径

三、代码：

public class Solution {
private int count = 0;
public static int com(TreeNode root, int sum) {
if (root != null) {
if (root.val == sum) {
return 1 + com(root.left, 0) + com(root.right, 0);
} else {
return com(root.left, sum - root.val) + com(root.right, sum - root.val);
}
}

return 0;
}
public int pathSum(TreeNode root, int sum) {
if (root == null) {
return count;
}
int tmp = com(root, sum);
if(tmp != 0){
count += tmp;
}
pathSum(root.left, sum);
pathSum(root.right, sum);
return count;
}

}