[数据结构与算法]437 路径总和-02

437 路径总和-02

前缀和，dfs，回溯

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int count=0;
    Map<TreeNode,Integer> map=new HashMap<>();
    public int pathSum(TreeNode root, int targetSum) {
        dfs(root,targetSum,0);

        return count;
    }
    
    public void dfs(TreeNode root,int targetSum,int currSum){
        if (root==null){
            return;
        }
        currSum+=root.val;
        
        
        for (Map.Entry<TreeNode,Integer> entry:map.entrySet()){
            if (currSum-entry.getValue()==targetSum){
                count++;
            }
        }
        if(currSum==targetSum){
            count++;
        }
        map.put(root,currSum);
        dfs(root.left,targetSum,currSum);
        dfs(root.right,targetSum,currSum);
        
        map.remove(root);
        
    }
    
}

时间复杂度$O(n)$，$n$是二叉树的节点个数。
空间复杂度$O(h)$，$h$是二叉树的深度，使用哈希表。