Update 315. Count of Smaller Numbers After Self.md

solutions/315. Count of Smaller Numbers After Self.md

@@ -23,11 +23,14 @@
 
 此题还可以用线段树/树状数组做，我们知道线段树和树状数组可以求前缀和，而这题可以转换成求前缀和。具体转换过程如下：
 
-1. 我们先遍历一遍数组，确定数组中元素的最小值`MIN`和`MAX`，然后想象有一个大小为`MAX - MIN`的全0数组`arr`，在这个数组上构建线段树/树状数组；
+1. 我们先遍历一遍数组，确定数组中元素的最小值`MIN`和`MAX`，然后想象有一个大小为`MAX - MIN`的全0数组`arr`，`arr[i]=j`表示`i+MIN`出现了j次。然后在这个数组上构建线段树/树状数组；
-2. 然后从后往前遍历数组nums，将`arr[nums[i]]++`，更新线段树/树状数组，这样`arr`在区间`(nums[i], MAX]`的元素和即为`nums[i]`右侧它小的元素个数。
+2. 然后从后往前遍历数组nums，将`arr[nums[i]-MIN]++`，表示出现次数加1，更新线段树/树状数组，这样就可很方便求得右侧比它小的元素个数。
 
 时间复杂度O(nlogN)，空间复杂度O(N)，其中`N = MAX - MIN`；
 
+关于线段树和树状数组可参考我的博客[Range Sum Query - Mutable (区间查询)](https://tangshusen.me/2019/11/17/range-sum-query-mutable/)
+
+
 # C++
 ## 思路一
 ``` C++
@@ -119,6 +122,71 @@ public:
     }
 };
 ```
+## 思路三、线段树
+``` C++
+class SegTree{
+private:
+    int n; 
+    vector<int>tree; // 用一个长为2n的数组来表示树
+public:
+    SegTree(vector<int>& nums) {
+        /* 对数组nums建线段树, 方便求sum(nums[i,...,j])以及更新nums[i]*/
+        n = nums.size();
+        tree = vector<int>(n*2, 0); 
+        
+        for(int i = 0; i < n; i++) // 建树
+            update(i, nums[i]);
+    }
+    
+    void update(int i, int diff) {
+        /*更新操作: 将nums[i]的值加上diff*/
+        i += n; // 转换为线段树下标
+        while(i > 0){
+            tree[i] += diff;
+            i >>= 1;
+        }
+    }
+    
+    int sumRange(int i, int j) {
+        /**求nums[i,...,j]的和*/
+        if(i > j) return 0;
+        
+        i += n; j += n; // 转换为线段树下标
+        int res = 0;
+        for(; i <= j; i >>= 1, j >>=1){
+            if(i & 1) res += tree[i++]; // 是右孩子
+            if(!(j & 1)) res += tree[j--];
+        }
+        return res;
+    }
+};
+class Solution {
+public:
+    vector<int> countSmaller(vector<int>& nums) {
+        int n = nums.size();
+        vector<int>res(n, 0);
+        if(!n) return res;
+        
+        int min_num = nums[0], max_num = nums[0];
+        for(int &num: nums){
+            min_num = min(min_num, num);
+            max_num = max(max_num, num);
+        }
+        
+        // nums_for_SegTree[i] = j 表示数字i出现了j次, 初始全0次
+        vector<int>nums_for_SegTree(max_num - min_num + 1, 0);
+
+        SegTree st = SegTree(nums_for_SegTree);
+          
+        for(int i = n - 1; i >= 0; i--){
+            res[i] = st.sumRange(0, nums[i] - min_num - 1);
+            st.update(nums[i] - min_num, 1); // 出现次数+1, 更新树
+        }
+        
+        return res;
+    }
+};
+```
 
 ## 思路三、树状数组
 [来源](https://leetcode-cn.com/problems/count-of-smaller-numbers-after-self/solution/c-shu-zhuang-shu-zu-by-mryx/)
@@ -164,108 +232,3 @@ public:
     }
 };
 ```
-
-## 思路三、线段树
-[来源](https://leetcode-cn.com/problems/count-of-smaller-numbers-after-self/solution/c-xian-duan-shu-jie-fa-by-dufre/)
-
-``` C++
-struct SegmentTreeNode{
-        int start;
-        int end;
-        int count;
-
-        SegmentTreeNode* left;
-        SegmentTreeNode* right;
-
-        SegmentTreeNode(int _start, int _end):start(_start),end(_end) {
-            count = 0;
-            left = NULL;
-            right = NULL;
-        }
-};
-
-class Solution {
-public:
-    SegmentTreeNode* build(int start, int end){
-        if (start > end)
-            return NULL;
-        
-        SegmentTreeNode* root = new SegmentTreeNode(start, end);
-
-        if (start == end){
-            root->count = 0;
-        }else{
-            int mid = start + (end - start)/2;
-            root->left = build(start, mid);
-            root->right = build(mid+1, end);
-        }
-        return root;
-    }
-
-    int count(SegmentTreeNode* root, int start, int end){
-        if (root == NULL || start>end)
-            return 0;
-        if (start==root->start && end==root->end){
-            return root->count;
-        }
-        int mid = root->start + (root->end - root->start)/2;
-        int leftcount = 0, rightcount = 0;
-
-        if (start <= mid){
-            if (mid < end)
-                leftcount = count(root->left, start, mid);
-            else
-                leftcount = count(root->left, start, end);
-        }
-
-        if (mid < end){
-            if (start <= mid)
-                rightcount = count(root->right, mid+1, end);
-            else
-                rightcount = count(root->right, start, end);
-        }
-
-        return (leftcount + rightcount);
-    }
-
-    void insert(SegmentTreeNode* root, int index, int val){
-        if (root->start==index && root->end==index){
-            root->count += val;
-            return;
-        }
-
-        int mid = root->start + (root->end - root->start)/2;
-        if (index>=root->start && index<=mid){
-            insert(root->left, index, val);
-        }
-        if (index>mid && index<=root->end){
-            insert(root->right, index, val);
-        }
-
-        root->count = root->left->count + root->right->count;
-    }
-
-    vector<int> countSmaller(vector<int>& nums) {
-        vector<int> res;
-        if (nums.empty())
-            return res;
-        res.resize(nums.size());
-        int start = nums[0];
-        int end = nums[0];
-
-        for (int i=1; i<nums.size(); i++){
-            start = min(start, nums[i]);
-            end = max(end, nums[i]);
-        }
-
-        SegmentTreeNode* root = build(start, end);
-
-        for (int i=nums.size()-1; i>=0; i--){
-            res[i] = count(root, start, nums[i]-1);
-            insert(root, nums[i], 1);
-        }  
-
-        return res;      
-    }
-};
-```