LeetCode 315. Count of Smaller Numbers After Self

原题链接在这里：https://leetcode.com/problems/count-of-smaller-numbers-after-self/

题目：

You are given an integer array nums and you have to return a new counts array. The counts array has the property where counts[i] is the number of smaller elements to the right of nums[i].

Example:

Given nums = [5, 2, 6, 1]

To the right of 5 there are 2 smaller elements (2 and 1).
To the right of 2 there is only 1 smaller element (1).
To the right of 6 there is 1 smaller element (1).
To the right of 1 there is 0 smaller element.

Return the array [2, 1, 1, 0].

题解：

从右向左扫描数组nums, try to find the position of nums[i] in BST.

For each BST node, it contains its left subtree size count, its duplicate count.

When inserting a new node, returns the sum of smaller count. 

Time Complexity: O(n^2). BST不一定balance. Space: O(n).

AC Java:

class Solution {
    public List<Integer> countSmaller(int[] nums) {
        LinkedList<Integer> res = new LinkedList<>();
        if(nums == null || nums.length == 0){
            return res;
        }
        
        int n = nums.length;
        res.offerFirst(0);
        TreeNode root = new TreeNode(nums[n - 1]);
        root.count = 1;
        
        for(int i = n - 2; i >= 0; i--){
            int smallerCount = insert(root, nums[i]);
            res.offerFirst(smallerCount);
        }
        
        return res;
    }
    
    private int insert(TreeNode root, int num){
        int smallerCountSum = 0;
        while(root.val != num){
            if(root.val > num){
                root.leftCount++;
                if(root.left == null){
                    root.left = new TreeNode(num);
                }
                
                root = root.left;
            }else{
                smallerCountSum += root.leftCount + root.count;
                if(root.right == null){
                    root.right = new TreeNode(num);
                }
                
                root = root.right;
            }
        }
        
        root.count++;
        return smallerCountSum + root.leftCount;
    }
}

class TreeNode{
    int val;
    int count;
    int leftCount;
    TreeNode left;
    TreeNode right;
    
    public TreeNode(int val){
        this.val = val;
        this.count = 0;
        this.leftCount = 0;
    }
}

类似Reverse Pairs.