Problem 1: Invert Binary Tree

Link to the problem: https://leetcode.com/problems/invert-binary-tree/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root==null)
            return null;
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        invertTree(root.left);
        invertTree(root.right);
        return root;        
    }
}

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Problem 2: Valid Palindrome

Link to the problem: https://leetcode.com/problems/valid-palindrome/

class Solution {
    public boolean isPalindrome(String s) {

        s = s.toLowerCase();
        s=s.trim();
        s=s.replaceAll("[^a-zA-Z0-9]", "");

        int i=0;
        int j=s.length()-1;

        while(i<=j)
            if(s.charAt(i++)!=s.charAt(j--)) return false;

        return true;
    }
}

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Problem 3: Binary Search

Link to the problem: https://leetcode.com/problems/binary-search/

class Solution {
    public int search(int[] nums, int target) {
        int low=0;
        int high = nums.length-1;
        while(low<=high){
            int mid = low+(high-low)/2;
            if(nums[low]==target)
                return low;
            else if(nums[high]==target)
                return high;
            else if(nums[mid]==target)
                return mid;
            else if(nums[mid]<target)
                low=mid+1;
            else
                high=mid-1;
        }
        return -1;
    }
}

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Problem 4: Ransom Note

Link to the problem: https://leetcode.com/problems/ransom-note/

class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
        Map<Character, Integer> map = new HashMap<>();
        for(char ch:magazine.toCharArray()){
            if(map.containsKey(ch))
                map.put(ch, map.get(ch)+1);
            else
                map.put(ch, 1);
        }
        for(char ch:ransomNote.toCharArray()){
            if(map.containsKey(ch)){
                if(map.get(ch)==1)
                    map.remove(ch);
                else
                    map.put(ch, map.get(ch)-1);
            }else 
                return false;
        }
        return true;
    }
}

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Problem 5: Climbing Stairs

Link to the problem: https://leetcode.com/problems/climbing-stairs/

class Solution {
    public int climbStairs(int n) {
        if(n==1)
            return 1;
        if(n==2)
            return 2;
        int num1 = 1;
        int num2 = 2;
        while(n-->2){
            int temp = num2;
            num2 += num1;
            num1 = temp;
        }
        return num2;
    }
}

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