LeetCode_437. Path Sum III

Posted 2021-03-24 邓戈麟

 

437. Path Sum III

Easy

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

 

package leetcode.easy;

/**
 * Definition for a binary tree node. public class TreeNode { int val; TreeNode
 * left; TreeNode right; TreeNode(int x) { val = x; } }
 */
public class PathSumIII {
	int count = 0;

	private void helper(TreeNode root, int sum) {
		if (root == null) {
			return;
		}

		if (root.val == sum) {
			count++;
		}

		if (root.left != null) {
			helper(root.left, sum - root.val);
		}

		if (root.right != null) {
			helper(root.right, sum - root.val);
		}
	}

	public int pathSum(TreeNode root, int sum) {
		if (root == null) {
			return count;
		}
		helper(root, sum);
		if (root.left != null) {
			pathSum(root.left, sum);
		}
		if (root.right != null) {
			pathSum(root.right, sum);
		}
		return count;
	}

	@org.junit.Test
	public void test() {
		int sum = 8;
		TreeNode tn11 = new TreeNode(10);
		TreeNode tn21 = new TreeNode(5);
		TreeNode tn22 = new TreeNode(-3);
		TreeNode tn31 = new TreeNode(3);
		TreeNode tn32 = new TreeNode(2);
		TreeNode tn34 = new TreeNode(11);
		TreeNode tn41 = new TreeNode(3);
		TreeNode tn42 = new TreeNode(-2);
		TreeNode tn44 = new TreeNode(1);
		tn11.left = tn21;
		tn11.right = tn22;

		tn21.left = tn31;
		tn21.right = tn32;
		tn22.left = null;
		tn22.right = tn34;

		tn31.left = tn41;
		tn31.right = tn42;
		tn32.left = null;
		tn32.right = tn44;
		tn34.left = null;
		tn34.right = null;

		tn41.left = null;
		tn41.right = null;
		tn42.left = null;
		tn42.right = null;
		tn44.left = null;
		tn44.right = null;
		System.out.println(pathSum(tn11, sum));
	}
}