LeetCode [437] Path Sum III

437. Path Sum III

Medium

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

/**
 * 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 ret = 0;
    //prefixSum -> ways to get prefixSum in one path (root -> node, excluding the current node)
    Map<Integer, Integer> ways = new HashMap<>();
    public int pathSum(TreeNode root, int sum) {
        ways.put(0,1);
        helper(root, sum, 0);
        return ret;
    }
    
    //compute ways adding to "sum" ending at each node
    void helper(TreeNode node, int sum, int cur){
        if(node == null) return;
        cur += node.val;
        ret += ways.getOrDefault(cur-sum, 0);
        
        ways.put(cur, ways.getOrDefault(cur, 0)+1);
        helper(node.left, sum, cur);
        helper(node.right, sum, cur);
        
        //when exiting a branch, it swithes to another path (eg., left to right); therefore, we need to restore "ways"
        ways.put(cur, ways.get(cur)-1);
    }
}