Backtracking-3 Done

Leetcode_51.java

@@ -0,0 +1,88 @@
+//Backtracking
+//Declare a grid - n*n
+//Check the possibilty by giving first row first element
+//Helper recurssion start with first row first element
+//It will check whether it is a valid or not
+//If valid, mark as true and go to second row 
+//Check for the valid block. If valid go to third row and so on
+//At any place where the block is not valid, backtrack. Means make that block of grid as false
+//As a base case, add the path to result
+//TC: n!
+//SC: O( n^2 ) for the board + O( n ) recursive stack space
+class Solution {
+    public List<List<String>> solveNQueens(int n) {
+        List<List<String>> ans=new ArrayList<>();
+        boolean[][] grid=new boolean[n][n];
+        helper(grid,0,ans);
+        return ans;
+
+
+    }
+
+    private void helper(boolean[][] grid,int i,List<List<String>> result){
+        //base case
+        if(i==grid.length){
+            List<String> curr=new ArrayList<>();
+            for(int r=0;r<grid.length;r++){
+                StringBuilder sb=new StringBuilder();
+                for(int c=0;c<grid[0].length;c++){
+                    if(grid[r][c]==true){
+                        sb.append('Q');
+                    }else{
+                        sb.append('.');
+                    }
+                }
+                curr.add(sb.toString());
+            }
+            result.add(curr);
+            return;
+        }
+
+
+        //actual logic
+        for(int j=0;j<grid.length;j++){
+            if(isValid(grid,i,j)){
+                grid[i][j]=true;
+                helper(grid,i+1,result);
+                grid[i][j]=false;
+
+            }
+
+        }
+    }
+
+    private boolean isValid(boolean[][] grid,int i,int j){
+        int r=0;
+        while(r>=0 && r<i){
+            if(grid[r][j]==true){
+                return false;
+            }
+            r+=1;
+        }
+
+        r=i-1;
+        int c=j-1;
+
+        while(r>=0 && c>=0){
+            if(grid[r][c]==true){
+                return false;
+            }
+            r-=1;
+            c-=1;
+        }
+
+        r=i-1;
+        c=j+1;
+        while(r>=0 && c<grid.length){
+            if(grid[r][c]==true){
+                return false;
+            }
+            r-=1;
+            c+=1;
+        }
+
+        return true;
+
+
+    }
+}

Leetcode_79.java

@@ -0,0 +1,54 @@
+//Have a dir array
+//Call helper from every index
+//In helper - check all neighbour blocks with index array
+//If mataches with word characters - return true
+//TC: O(m * n * 4^L) where L = word.length()
+//SC: O(d)
+
+
+class Solution {
+    int[][] dir;
+    boolean flag;
+    public boolean exist(char[][] board, String word) {
+        this.dir=new int[][]{{1,0},{-1,0},{0,1},{0,-1}};
+        this.flag=false;
+        for(int i=0;i<board.length;i++){
+            for(int j=0;j<board[0].length;j++){
+                if (flag) return true; 
+                helper(board,word,0,i,j);
+            }
+        }
+
+        return flag;
+
+
+
+    }
+    private void helper(char[][] board,String word, int index, int r, int c){
+        if(flag) return;
+
+        //base logic
+        if(r<0 || c<0 || r>=board.length || c>=board[0].length){
+            return;
+        }
+        if(board[r][c]!=word.charAt(index)) return;
+
+        if(index==word.length()-1){
+            flag=true;
+            return;
+        }
+
+
+        board[r][c]='#';
+
+        //actual logic
+        for(int[] d:dir){
+            int nr=r+d[0];
+            int nc=c+d[1];
+
+            helper(board,word,index+1,nr,nc);
+
+        }
+        board[r][c]=word.charAt(index);
+    }
+}