为什么“ res”在我的DFS功能中没有改变？

Question

I have the following code for computing the depth of a tree:

class Solution {
    public int maxDepth(TreeNode root) {
        int res = 0;
        DFS(root, res, 1);
        return res;
    }
    private void DFS(TreeNode root, int res, int curDepth) {
        if (root == null) {
            return; 
        }

        if (curDepth > res) {
            res = curDepth;
        }
        DFS(root.left, res, curDepth + 1);
        DFS(root.right, res, curDepth + 1);
    }
}

If I give it the input [3, 9, 20, null, null, 15, 7] , I'd expect to have res equal 3, as it represents the depth of the binary tree. But res ends up being equal to zero. It seems that it never changed.

Does anyone know why?

Answer 1

Remember that in the language you're using (I think it's Java?) that parameters to functions are passed by value . That means that when you write

DFS(root, res, 1)

you're not actually taking the variable res and handing it off to the DFS function to modify. Instead, you're saying "make a copy of whatever value happens to be stored in the variable called res , then hand that copy over to DFS ."

The method DFS has a parameter called res , but the fact that it's called res and that you have your own variable called res up in maxDepth doesn't mean that those values are linked. As an example, I once taught I class with six students in it all named Zoe (true story!) Although all of them were named Zoe, they were definitely not the same person as one another! I couldn't talk to one person name Zoe and expect all the other Zoes to have any inkling of what we talked about. In the same way, your res variable in maxDepth is completely separate from the res variable in DFS . They coincidentally have the same first name, but they're not the same person.

There are a number of ways you could fix this, and probably the best way to do this would be to have DFS return a value to you, and then write your code like this:

public int maxDepth(TreeNode root) {
    return DFS(root, 1);
}

private int DFS(TreeNode root, int currDepth) {
    // For you to figure out!
}

Returning a value from a function is the preferred way to get information out of one method and into another. So now what you'll need to do is think about what value you'd like to return from DFS and what updates you'll need to make to it to get that information to propagate properly.

Answer 2

If you want to calculate the height of a node you need simply to add one to the maximum of the height of the left and right tree. If a node does not exist it has no height.

private int height(TreeNode node) {
    if (node == null) {
        return 0;
    }
    return 1 + Math.max(height(node.left), height(node.right));
}

The reason for your solution not to work is that the value of res is passed by value like templatetypedef said.

Actually you do not need a parameter with the depth to solve this problem.