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2,array

(1) How to implement Java ArrayList

 Public classArrayList {Private intcapacity; Private intsize; Private int[] data;  PublicArrayList (intcapacity_) {Capacity=capacity_; Size= 0; Data=New int[capacity]; }     Public intGetintindex) {        if(Index < 0 | | | Index >=size) {            //Throw ExceptionSystem.out.println ("ArrayIndexOutOfBoundsException:" +index); }        returnData[index]; }     Public voidSetintIndexintvalue) {        if(Index < 0 | | | Index >=size) {System.out.println ("ArrayIndexOutOfBoundsException:" +index); } Data[index]=value; }     Public voidAddintIndexintvalue) {        if(Index < 0 | | index >size) {System.out.println ("ArrayIndexOutOfBoundsException:" +index); }        if(Size = =capacity)        {Resize (); } size++;  for(inti = size-1; I >= index + 1; --i) {Data[i]= Data[i-1]; } Data[index]=value; }    Private voidResize () {capacity*= 2; int[] TempData =New int[size];  for(inti = 0; i < size; ++i) {Tempdata[i]=Data[i]; } Data=New int[capacity];  for(inti = 0; i < size; ++i) {Data[i]=Tempdata[i]; }    }     Public voidRemoveintindex) {        if(Index < 0 | | | Index >=size) {System.out.println ("ArrayIndexOutOfBoundsException:" +index); } size--;  for(inti = index; i < size; ++i) {Data[i]= Data[i + 1]; }    }         Public Static voidMain (string[] args) {int[] A = {1}; System.out.println (a[-1]); }}

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(2) Sum

1. The Sum My submissions questioneditorial solutiontotal Accepted:247444 Total submissions:1017480Difficulty:easygiven An array of integers,returnindices of the numbers such that they add up to a specific target. You may assume this each input would has exactly one solution. Example:given Nums= [2, 7, one, all], target = 9, Because nums[0] + nums[1] = 2 + 7 = 9,return[0, 1]. UPDATE (2016/2/13): thereturnFormat had been changed to zero-based indices. Please read the above updated description carefully. Subscribe to see which companies asked ThisQuestion

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Analyse:use the HashMap to save (Target-nums[i], i)

 Public classSolution { Public int[] Twosum (int[] Nums,inttarget) {        int[] res =New int[2]; if(NULL= = Nums | | Nums.length = = 0) {            returnRes; } HashMap<integer, integer> map =NewHashMap ();  for(inti = 0; i < nums.length; ++i) {if(!Map.containskey (Nums[i])) {Map.put (target-Nums[i], i); } Else{res[0] =Map.get (Nums[i]); res[1] =i;  Break; }        }        returnRes; }}/*** All Error: * 1, Compile error:line 20:error:unclosed comment. Because the space of ' * ' and '/' * 2, Map.add. Carelessness*/

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1,time Complexity:

if the recurrence equation is T (n) = at (b/n) + N ^ c.  if logb a > C, t (n) = n ^ (logb a)if logb a = = C, t (n) = n ^ c * Lognif
     LOGB A < C, T (n) = n ^ C

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