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# 110 Balanced Binary Tree

## 110. Balanced Binary Tree

## 1. Question

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees ofeverynode never differ by more than 1.

## 2. Implementation

(1) DFS

```java
class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) {
            return true;
        }

        return dfs(root) != -1;
    }

    public int dfs(TreeNode node) {
        if (node == null) {
            return 0;
        }

        int leftHeight = dfs(node.left);
        int rightHeight = dfs(node.right);

        if (leftHeight == -1 || rightHeight == -1 || Math.abs(leftHeight - rightHeight) > 1) {
            return -1;
        }
        return Math.max(leftHeight, rightHeight) + 1;
    }
}
```

## 3. Time & Space Complexity

DFS: 时间复杂度: O(n), 空间复杂度: O(h)