LeetCode 437.路径总和III

​​https://leetcode-cn.com/problems/path-sum-iii/solution/javajie-fa-shi-jian-100-kong-jian-93-by-xiao-chao-/​​

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
1.递归：
总路径数目 = 以从根结点出发的路径数目+以从左节点出发的路径数目+以从右节点出发的路径数目
class Solution {
  
    public int pathSum(TreeNode root, int sum) {
        if(root==null){
            return 0;
        }
        return findPath(root,sum)
        +pathSum(root.left,sum)
        +pathSum(root.right,sum);
        
    }
     public int   findPath(TreeNode root, int sum) {
            if(root==null){
                return 0;
            }
            //sum为0时，就刚好找到了该路径！
            sum-=root.val;
            
        int left = findPath(root.left,sum);
        int right = findPath(root.right,sum);
        return sum==0?left+right+1:left+right;
    }
}