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)
寫(xiě)錯(cuò)好幾次...注意單個(gè)節(jié)點(diǎn)也算一條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);
}
}
