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Backtracking.java
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193 lines (173 loc) · 5.25 KB
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/*
//backtracking in arrays.... tc= O(n)=sc
public class Backtracking {
public static void changeArr(int arr[], int i, int val){
//base case
if(i== arr.length){
printArr(arr);
return;
}
//recursion
arr[i]=val;
changeArr(arr, i+1, val+1);//fnx call step
arr[i]=arr[i]-2;//backtracking
}
public static void printArr(int arr[]){
for(int i=0;i<arr.length;i++){
System.out.print(arr[i]+" ");
}
System.out.println();
}
public static void main(String[] args) {
int arr[]= new int[5];
changeArr(arr, 0, 1);
printArr(arr);
}
}
*/
/*
//find subset
// import java.util.ArrayList;
// import java.util.List;
public class Backtracking {
public static void findSubset(String str,int i,String ans){
if(i==str.length()){
if(ans.length() == 0){
System.out.println("null");
}
else{
System.out.println(ans);
}
return;
}
//yes
findSubset(str, i+1, ans+str.charAt(i));
//no
findSubset(str, i+1, ans);
}
// public List<List<Integer>> subsets(int[] nums) {
// List<List<Integer>> result = new ArrayList<>();
// findSubsets(0, nums, new ArrayList<>(), result);
// return result;
// }
// private void findSubsets(int index, int[] nums, List<Integer> current, List<List<Integer>> result) {
// if (index == nums.length) {
// result.add(new ArrayList<>(current));
// return;
// }
// // Include nums[index]
// current.add(nums[index]);
// findSubsets(index + 1, nums, current, result);
// // Backtrack (remove last added element)
// current.remove(current.size() - 1);
// // Exclude nums[index]
// findSubsets(index + 1, nums, current, result);
// }
public static void main(String[] args) {
String str = "abc";
findSubset(str, 0, "");
}
}*/
/*
//permutation in string
public class Backtracking {
public static void findPermutation(String str,String ans){
//base case
if(str.length()==0){
System.out.println(ans);
return;
}
//recursion
for(int i=0;i< str.length();i++){
char curr = str.charAt(i);
//"abcde"--> "ab"+"de"
String newStr =str.substring(0,i)+str.substring(i+1);
findPermutation(newStr, ans+curr);
}
}
public static void main(String[] args) {
String str = "abc";
findPermutation(str, "");
}
}
*/
/* n queens problem......
public class Backtracking {
public static boolean isSafe(char board[][], int row, int col) {
//verticalk up
for (int i = row - 1; i >= 0; i--) {
if (board[i][col] == 'Q') {
return false;
}
}
//dig leftup
for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--) {
if (board[i][j] == 'Q') {
return false;
}
}
//diag right up
for (int i = row - 1, j = col + 1; i >= 0 && j < board.length; i--, j++) {
if (board[i][j] == 'Q') {
return false;
}
}
return true;
}
public static void nQeens(char board[][], int row) {
//base
if (row == board.length) {
printBoard(board);
return;
}
//colamn loop
for (int j = 0; j < board.length; j++) {
if (isSafe(board, row, j)) {
board[row][j] = 'Q';
nQeens(board, row + 1);//function
board[row][j] = 'x';//backtracking
}
}
}
public static void printBoard(char board[][]) {
System.out.println(".......chess board........");
for (int i = 0; i < board.length; i++) {
for (int j = 0; j < board.length; j++) {
System.out.print(board[i][j] + " ");
}
System.out.println();
}
}
public static void main(String[] args) {
int n = 4;
char board[][] = new char[n][n];
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
board[i][j] = 'x';
}
}
nQeens(board, 0);
}
}
*/
//subset
class Solution {
public List<List<Integer>> subsets(int[] nums) {
List<List<Integer>> result = new ArrayList<>();
findSubsets(0, nums, new ArrayList<>(), result);
return result;
}
private void findSubsets(int index, int[] nums, List<Integer> current, List<List<Integer>> result) {
if (index == nums.length) {
result.add(new ArrayList<>(current));
return;
}
// Include nums[index]
current.add(nums[index]);
findSubsets(index + 1, nums, current, result);
// Backtrack (remove last added element)
current.remove(current.size() - 1);
// Exclude nums[index]
findSubsets(index + 1, nums, current, result);
}
}