c++设计算法求二叉树的结点个数，深度，并按前序次序打印二叉树中的叶子结点，并求树中结点x的第i个孩子

以下是C++的实现：

#include<iostream>
using namespace std;

// 树节点的结构
struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

// 获取二叉树的结点个数
int getNodeNum(TreeNode* root) {
    if (root == NULL) {
        return 0;
    }
    return 1 + getNodeNum(root->left) + getNodeNum(root->right);
}

// 获取二叉树的深度
int getDepth(TreeNode* root) {
    if (root == NULL) {
        return 0;
    }
    int left_depth = getDepth(root->left);
    int right_depth = getDepth(root->right);
    return max(left_depth, right_depth) + 1;
}

// 按前序次序打印二叉树中的叶子结点
void printLeafNodes(TreeNode* root) {
    if (root == NULL) {
        return;
    }
    if (root->left == NULL && root->right == NULL) {
        cout << root->val << " ";
    }
    printLeafNodes(root->left);
    printLeafNodes(root->right);
}

// 获取树中结点x的第i个孩子
TreeNode* getIthChild(TreeNode* root, int x, int i) {
    if (root == NULL) {
        return NULL;
    }
    if (root->val == x) {
        if (i == 1) {
            return root->left;
        } else if (i == 2) {
            return root->right;
        } else {
            return NULL;
        }
    }
    TreeNode* child = getIthChild(root->left, x, i);
    if (child != NULL) {
        return child;
    }
    return getIthChild(root->right, x, i);
}

int main() {
    // 构建一个二叉树
    TreeNode* root = new TreeNode(1);
    root->left = new TreeNode(2);
    root->right = new TreeNode(3);
    root->left->left = new TreeNode(4);
    root->left->right = new TreeNode(5);
    root->right->left = new TreeNode(6);
    root->right->right = new TreeNode(7);

    // 获取二叉树的结点个数和深度
    cout << "Node num: " << getNodeNum(root) << endl;
    cout << "Depth: " << getDepth(root) << endl;

    // 按前序次序打印二叉树中的叶子结点
    cout << "Leaf nodes: ";
    printLeafNodes(root);
    cout << endl;

    // 获取结点2的第1个孩子
    TreeNode* child = getIthChild(root, 2, 1);
    if (child != NULL) {
        cout << "Node 2, child 1: " << child->val << endl;
    } else {
        cout << "Node 2, child 1 not found" << endl;
    }

    return 0;
}

输出结果为：

Node num: 7
Depth: 3
Leaf nodes: 4 5 6 7
Node 2, child 1: 4