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979. Distribute Coins in Binary Tree 👍

  • Time: $O(n)$
  • Space: $O(h)$
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class Solution {
 public:
  int distributeCoins(TreeNode* root) {
    int ans = 0;
    dfs(root, ans);
    return ans;
  }

 private:
  // Returns the number of coins I can give (positive) / take (negative).
  int dfs(TreeNode* root, int& ans) {
    if (root == nullptr)
      return 0;

    const int l = dfs(root->left, ans);
    const int r = dfs(root->right, ans);
    ans += abs(l) + abs(r);
    return (root->val - 1) + l + r;
  }
};

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class Solution {
  public int distributeCoins(TreeNode root) {
    dfs(root);
    return ans;
  }

  private int ans = 0;

  // Returns the number of coins I can give (positive) / take (negative).
  private int dfs(TreeNode root) {
    if (root == null)
      return 0;

    final int l = dfs(root.left);
    final int r = dfs(root.right);
    ans += Math.abs(l) + Math.abs(r);

    return (root.val - 1) + l + r;
  }
}