leetcode437--Path Sum III

https://leetcode.com/problems/path-sum-iii/

理解比较困难，可以先看https://www.cnblogs.com/albert67/p/10416402.html

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int pathSum(TreeNode root, int sum) {
        HashMap<Integer,Integer> preSum=new HashMap<>();
        preSum.put(0,1);// 当键值等于0，说明curSum==sum，所以把设为1
        return helper(root,0,sum,preSum);
    }
    
    public int helper(TreeNode n,int curSum,int target,HashMap<Integer,Integer> preSum){
        if(n==null)return 0;
        curSum+=n.val;
        int res=preSum.getOrDefault(curSum-target,0);
        preSum.put(curSum,preSum.getOrDefault(curSum,0)+1);
        res+=helper(n.left,curSum,target,preSum)+helper(n.right,curSum,target,preSum);
        preSum.put(curSum,preSum.get(curSum)-1);
        return res;
    }
}

递归比较慢，但很容易理解

class Solution {
    public int pathSum(TreeNode root, int sum) {
        if(root==null)return 0;
        return add(root,sum)+pathSum(root.left,sum)+pathSum(root.right,sum);
    }
    public int add(TreeNode n,int sum){
        if(n==null)return 0;
        return (sum-n.val==0?1:0)+add(n.left,sum-n.val)+add(n.right,sum-n.val);
    }
    
}