Leetcode Algos

public int climbStairs(int n) {
        int[] previousClimbs = new int[n + 1];
        previousClimbs[0] = 1;
        previousClimbs[1] = 1;
        for(int i = 2; i <= n; i++) {
            previousClimbs[i] = previousClimbs[i -1] + previousClimbs[i - 2];
        }
        
     return previousClimbs[n];
 }
public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1 == null)
            return t2;
        Stack < TreeNode[] > stack = new Stack < > ();
        stack.push(new TreeNode[] {t1, t2});
        while (!stack.isEmpty()) {
            TreeNode[] t = stack.pop();
            if (t[0] == null || t[1] == null) {
                continue;
            }
            t[0].val += t[1].val;
            if (t[0].left == null) {
                t[0].left = t[1].left;
            } else {
                stack.push(new TreeNode[] {t[0].left, t[1].left});
            }
            if (t[0].right == null) {
                t[0].right = t[1].right;
            } else {
                stack.push(new TreeNode[] {t[0].right, t[1].right});
            }
        }
        return t1;
    }
public int[] topKFrequent(int[] nums, int k) {
        ArrayList<LinkedList<Integer>> freqList = new ArrayList<>();
        for (int i = 0; i <= nums.length; i++) {
            freqList.add(new LinkedList<Integer>());
        }
        
        HashMap<Integer, Integer> elementFreq = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            if (elementFreq.containsKey(nums[i])){
                elementFreq.put(nums[i], elementFreq.get(nums[i]) + 1);
            }
            else {
                elementFreq.put(nums[i], 1);
            }
        }
        
        Iterator<Integer> elemItr = elementFreq.keySet().iterator();
        while (elemItr.hasNext()) {
            int currKey = elemItr.next();
            freqList.get(elementFreq.get(currKey)).add(currKey);
        }
        
        ArrayList<Integer> intsByFreq = new ArrayList<>();
        for (int i = 0; i < freqList.size(); i++) {            
            LinkedList<Integer> currList = freqList.get(i);
            ListIterator<Integer> currListItr = currList.listIterator(0);
            while (currListItr.hasNext()) {
                intsByFreq.add(currListItr.next());
            }
        }
        
        int[] topK = new int[k];
        int topKIdx = 0;
        for (int i = intsByFreq.size() - 1; i >= intsByFreq.size() - k; i--) {
            topK[topKIdx] = intsByFreq.get(i);
            topKIdx++;
        }
        
        return topK;
    }

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