437. Path Sum III

Description

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

tree

Return 3. The paths that sum to 8 are:

1. 5 -> 3
2. 5 -> 2 -> 1
3. -3 -> 11

Solution

DFS, time best O(nlogn) worst O(n ^ 2), space O(n)

写错好几次...注意单个节点也算一条path。

/**
 * 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) {  
        if (root == null) {
            return 0;
        }
        
        return pathSumFrom(root, sum) 
            + pathSum(root.left, sum) 
            + pathSum(root.right, sum);
    }
    
    public int pathSumFrom(TreeNode from, int sum) {
        if (from == null) {
            return 0;
        }

        return (from.val == sum ? 1 : 0)    // single node is also a valid path
            + pathSumFrom(from.left, sum - from.val) 
            + pathSumFrom(from.right, sum - from.val);
    }
}