adding lecture 20

leetcode/p1_twosum_hash_table.cpp

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+// leetcode problem 1; hash table solution, fast, but harder to write the code.
+
+// this table can have at most 1024 keys
+#define TABLE_SIZE 1024
+
+class Node {
+public:
+    int number;
+    int index;
+    Node* next;
+};
+
+// add this num-index mapping into the table
+// this has to be Node** table, because it's not just one pointer, it's an array of pointers.
+void add(int num, int index, Node** table){
+    int key;
+    key = abs(num%TABLE_SIZE);   // key will be something in between 0 and (TABLE_SIZE-1); num can be negative
+    if(table[key]==NULL){
+        // create the first node for this linked list
+        Node* node;
+        node = new Node;
+        node->number = num;
+        node->index = index;
+        node->next = NULL;
+        table[key] = node;
+    }else{
+        // insert a node to the beginning of this linked list
+        Node* node;
+        node = new Node;
+        node->number = num;
+        node->index = index;
+        node->next = table[key];
+        table[key] = node;
+    }
+}
+
+// give me the number, return the index
+int get(int num, Node** table){
+    int key = abs(num%TABLE_SIZE);   // key will be something in between 0 and (TABLE_SIZE-1); num can be negative
+    Node* node;
+    node = table[key];
+    while(node!=NULL){
+        if(node->number == num){
+            return node->index;
+        }
+        node = node->next;
+    }
+    // not found
+    return -1;
+}
+
+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+        // initialize it to something invalid
+        int index = -1;
+        int size = nums.size();
+        vector<int> ans;
+        // an array of pointers, each pointer points to the first node of a linked list.
+        Node* table[TABLE_SIZE];
+        for(int i=0;i<TABLE_SIZE;i++){
+            table[i]=NULL;
+        }
+        for(int i=0;i<size;i++){
+           index = get(target-nums[i], table);
+            // can't use the same element twice.
+            if(index==i){
+                continue;
+            }
+            if(index>=0){
+                ans.push_back(i);
+                ans.push_back(index);
+            }
+            // add an entry to the table
+            add(nums[i], i, table);
+        }
+        return ans;
+    }
+};