110. 平衡二叉树

C代码：DFS

int dfs (struct TreeNode* root) {
    if (NULL == root) {
        return 0;
    }
    int leftDepth = dfs(root->left);
    int rightDepth = dfs(root->right);
    if (fabs(leftDepth - rightDepth) > 1 || leftDepth == -1 || rightDepth == -1) {
        return -1;
    }
    return fmax(leftDepth, rightDepth) + 1;
}

bool isBalanced(struct TreeNode* root){
    return dfs(root) >= 0;
}