Next Greater Element I

class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        // M1 Stack & HashMap
        HashMap<Integer,Integer> hm = new HashMap<>();
        Stack<Integer> stack = new Stack<>();
        for (int n:nums2) {
            while (!stack.isEmpty() && stack.peek()<n) {
                hm.put(stack.pop(),n);
            }
            stack.push(n);
        }
        
        for (int i=0;i<nums1.length;i++) {
            if (hm.containsKey(nums1[i])) {nums1[i] = hm.get(nums1[i]);}
            else {nums1[i] = -1;}
        }
        return nums1;
    }
}

class Solution {
    public int[] nextGreaterElement(int[] nums1, int[] nums2) {
        // M2 HashMap
        HashMap<Integer,Integer> hm = new HashMap<>();
        int s2 = nums2.length;
        hm.put(nums2[s2-1],-1);
        for (int j=s2-2;j>=0;j--) {
            int n1 = nums2[j];
            int n2 = nums2[j+1];
            if (n1<n2) {
                hm.put(n1,n2);
            }
            else {
                int temp = hm.get(n2);
                while (temp < n1 && temp != -1) {
                    temp = hm.get(temp);
                }
                hm.put(n1,temp);
            }
            
        }
        int s1 = nums1.length;
        int[] out = new int[s1];
        for (int k=0;k<s1;k++) {
            out[k] = hm.get(nums1[k]);
        }
        return out;
    }
}

2022-10-08

Remove All Adjacent Duplicates In String

LeetCode Problem #1047

class Solution {
    public String removeDuplicates(String s) {

        // Using StringBuilder as Stack
        StringBuilder sb = new StringBuilder();
        int size = 0;
        for (char c: s.toCharArray()) {
            if (size != 0 && c == sb.charAt(size-1)) {
                sb.deleteCharAt(size-1);
                size--;
            }
            else {
                sb.append(c);
                size++;
                
            }
        }
        return sb.toString();
    }
}

class Solution {
    public String removeDuplicates(String s) {
        
        // Stack 
        Stack<Character> stack = new Stack<>();
        for (char c: s.toCharArray()) {
            if (!stack.isEmpty() && c == stack.peek()) {
                stack.pop();
            }
            else if (stack.isEmpty() || c != stack.peek()) {
                stack.push(c);
            }
        }
        StringBuilder sb = new StringBuilder();
        while (!stack.isEmpty()) {
            sb.insert(0,stack.pop());
        }
        return sb.toString();
    }
}