插入 20 后它挂在谁下面

还是那棵树，再插一个 20。运行下面这段程序：

#include <iostream>
#include <string>
#include <vector>
using namespace std;

struct BNode {
    int val;
    BNode* left = nullptr;
    BNode* right = nullptr;
};

BNode* insert(BNode* root, int val) {
    if (root == nullptr) return new BNode{val};
    if (val < root->val) root->left = insert(root->left, val);
    else if (val > root->val) root->right = insert(root->right, val);
    return root;
}

BNode* build() {
    BNode* root = nullptr;
    for (int v : {17, 24, 15, 13, 23}) root = insert(root, v);
    return root;
}

int main() {
    BNode* cur = build();
    while (true) {
        BNode* nxt = 20 < cur->val ? cur->left : cur->right;
        if (nxt == nullptr) break;
        cur = nxt;
    }
    cout << cur->val << endl;
}

（本题用 g++ -std=c++17 -O0 编译。）