437.Path-Sum-III

437. Path Sum III

题目地址

https://leetcode.com/problems/path-sum-iii/

题目描述

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

代码

Approach 1: DFS

path sum = 经过root节点的sum path + 左边root.left节点的sum path +右边root.right节点sum path

Time Complexity should be O(N^2) for the worst case and O(NlogN) for balanced binary Tree.

/**
 * 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;
    // 中 + 左 + 右
    return dfs(root, sum) + pathSum(root.left, sum) + pathSum(root.right, sum);
  }

  // 包含当前 node 的 path sum
  private int dfs(TreeNode node, int sum) {
    if (node == null)    return 0;

    return (node.val == sum ? 1 : 0) + 
      dfs(node.left, sum - node.val) + 
      dfs(node.right, sum - node.val);
  }
}

Approach #2 Prefix Sum + backtrace

class Solution {
    int count = 0;
    int k;
    HashMap<Integer, Integer> h = new HashMap();

    public int pathSum(TreeNode root, int sum) {
        k = sum;
        preorder(root, 0);
        return count;
    }

    public void preorder(TreeNode node, int currSum) {
        if (node == null)
            return;

        // current prefix sum
        currSum += node.val;

        // here is the sum we're looking for
        if (currSum == k)
            count++;

        // number of times the curr_sum − k has occured already, 
        // determines the number of times a path with sum k 
        // has occured upto the current node
        count += h.getOrDefault(currSum - k, 0);

        // add the current sum into hashmap
        // to use it during the child nodes processing
        h.put(currSum, h.getOrDefault(currSum, 0) + 1);

        // process left subtree
        preorder(node.left, currSum);
        // process right subtree
        preorder(node.right, currSum);

        // remove the current sum from the hashmap
        // in order not to use it during 
        // the parallel subtree processing
        h.put(currSum, h.get(currSum) - 1);
    }    

}