437. Path Sum III

Leetcode Tree

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

分析

深度优先搜索

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

private int pathSumFrom(TreeNode node, int sum) {
    if (node == null) return 0;
    return (node.val == sum ? 1 : 0) 
        + pathSumFrom(node.left, sum - node.val) + pathSumFrom(node.right, sum - node.val);
}