题目

count-of-smaller-numbers-after-self

2. 算法

* 二分查找

* 插入排序

* 二叉查找树

3. 代码

* 二分查找

// Binary Search
class Solution {
public:
    vector<int> countSmaller(vector<int>& nums) {
        vector<int> t, res(nums.size());
        for (int i = nums.size() - 1; i >= 0; --i) {
            int left = 0, right = t.size();
            while (left < right) {
                int mid = left + (right - left) / 2;
                if (t[mid] >= nums[i]) right = mid;
                else left = mid + 1;
            }
            res[i] = right;
            t.insert(t.begin() + right, nums[i]);
        }
        return res;
    }
};

* 插入排序

// Insert Sort
class Solution {
public:
    vector<int> countSmaller(vector<int>& nums) {
        vector<int> t, res(nums.size());
        for (int i = nums.size() - 1; i >= 0; --i) {
            int d = distance(t.begin(), lower_bound(t.begin(), t.end(), nums[i]));
            res[i] = d;
            t.insert(t.begin() + d, nums[i]);
        }
        return res;
    }
};

* 二叉查找树

// Binary Search Tree
class Solution {
public:
    struct Node {
        int val, smaller;
        Node *left, *right;
        Node(int v, int s) : val(v), smaller(s), left(NULL), right(NULL) {}
    };
    int insert(Node *&root, int v) {
        if (!root) return (root = new Node(v, 0)), 0;
        if (root->val > v) return root->smaller++, insert(root->left, v);
        else return insert(root->right, v) + root->smaller + (root->val < v ? 1 : 0);
    }
    vector<int> countSmaller(vector<int>& nums) {
        vector<int> res(nums.size());
        Node *root = NULL;
        for (int i = nums.size() - 1; i >= 0; --i) {
            res[i] = insert(root, nums[i]);
        }
        return res;
    }
};