437. Path Sum III.md

437. Path Sum III

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leetcode Daily Challenge on August 8th, 2020.

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Difficulty : Medium

Related Topics : Tree

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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

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Solution

• mine
  • Java
    • DFS Runtime: 27 ms, faster than 14.68%, Memory Usage: 40 MB, less than 30.10% of Java online submissions

      int ans = 0;
      public int pathSum(TreeNode root, int sum) {
          dfs(root, sum);
          return ans;
      }
      
      List<Integer> dfs(TreeNode node, int sum) {
          if (node == null) return null;
          List<Integer> left = new ArrayList<>();
          List<Integer> right = new ArrayList<>();
          if (node.left != null) left = dfs(node.left, sum);
          if (node.right != null) right = dfs(node.right, sum);
      
          List<Integer> res = new ArrayList<>();
          if (left.size() > 0 && right.size() > 0) {
              for (int i : left) {
                  res.add(i + node.val);
                  if (i + node.val == sum) ans++;
              }
              for (int i : right) {
                  if (i + node.val == sum) ans++;
                  res.add(i + node.val);
              }
          } else {
              left = left.size() == 0 ? right : left;
              for (int i : left) {
                  if (i + node.val == sum) ans++;
                  res.add(node.val + i);
              }
          }
          res.add(node.val);
          if (node.val == sum) ans++;
          return res;
      }
      

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• the most votes

• Runtime: 2 ms, faster than 100.00%, Memory Usage: 39.1 MB, less than 82.13% of Java online submissions

  public int pathSum(TreeNode root, int sum) {
      HashMap<Integer, Integer> preSum = new HashMap();
      preSum.put(0,1);
      return helper(root, 0, sum, preSum);
  }
  
  public int helper(TreeNode root, int currSum, int target, HashMap<Integer, Integer> preSum) {
      if (root == null) {
          return 0;
      }
  
      currSum += root.val;
      int res = preSum.getOrDefault(currSum - target, 0);
      preSum.put(currSum, preSum.getOrDefault(currSum, 0) + 1);
  
      res += helper(root.left, currSum, target, preSum) + helper(root.right, currSum, target, preSum);
      preSum.put(currSum, preSum.get(currSum) - 1);
      return res;
  }
  

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