Recursive processing - subproblem

First ： What is a subproblem
That is to put the overall problem , Can be concentrated
For example, look for the left and right of the tree
It can be regarded as looking One node The left node Right node

5. The detected value is value Does the element of exist

The recursive thought of subproblem ： The principle is to analyze only one root node

The principle is to analyze only one root node
The analysis is as follows ：
analysis A node
If A The value of the node is key
Then the return A node
Otherwise, traverse the left node
If the value returned by the left node is not empty Then return the return value
If the return value of the left node is null then Traverse the right node
If the return value of the right node is not empty Then return the return value
Otherwise, it will directly return null

//  The detected value is value Does the element of exist 
TreeNode find(TreeNode root, char val) {
    
    if(root == null) return null;
    if(root.val == val) {
    
        return root;
    }
    TreeNode ret1 = find(root.left,val);
    if(ret1 != null) {
    
        return ret1;
    }
    TreeNode ret2 = find(root.right,val);
    if(ret2 != null) {
    
        return ret2;
    }
    return null;
}