为什么相似解的运行时间相差这么大？

Question

还有一个problem in LeetCode. I use a straightforward recursive solution to solve it, but the run time is to long, which is 170ms. Then I found a similar solution，也是递归的，这个运行的时间也就10ms左右。为什么？

我的解决方案：

class Solution
{
public:
    bool isBalanced(TreeNode* root)
    {
        if (root == nullptr)
            return true;

        bool left = isBalanced(root->left);
        bool right = isBalanced(root->right);

        bool curr = false;
        int sub = height(root->left) - height(root->right);
        if (sub < 2 && sub > -2)
            curr = true;

        return left && right && curr;
    }

private:
    int height(TreeNode* root)
    {
        if (root == nullptr)
            return 0;

        int leftHeight = height(root->left);
        int rightHeight = height(root->right);
        return (leftHeight > rightHeight) ? (leftHeight + 1) : (rightHeight + 1);
    }
};

我找到的快速解决方案：

class Solution {
public:
    bool isBalanced(TreeNode *root) {
        if (root==NULL) return true;

        int left = treeDepth(root->left); 
        int right = treeDepth(root->right);

        if (left-right>1 || left-right < -1) {
            return false;
        }
        return isBalanced(root->left) && isBalanced(root->right);
    }

    int treeDepth(TreeNode *root) {
        if (root==NULL){
            return 0;
        }

        int left=1, right=1;

        left += treeDepth(root->left);
        right += treeDepth(root->right);

        return left>right?left:right;
    }

};

谢谢！

Answer 1

您的解决方案同时调用 isBalanced 和 height，总是。对于树中的每个节点。

更快的解决方案为每个节点调用 treeDepth，但提前退出并且如果知道树不平衡则不会调用 isBalanced。不调用不必要的 (recursive/expensive) 函数是一种优化。

为什么相似解的运行时间相差这么大？

Why the run time of similar solution is so different?

c++

recursion

tree-balancing