Array-3 Done

H-Index.java

@@ -0,0 +1,21 @@
+class Solution {
+    public int hIndex(int[] citations) {
+        Arrays.sort(citations);
+        int low = 0;
+        int high = citations.length - 1;
+
+        while(low <= high){
+            int mid = low + (high - low)/2;
+            int diff = citations.length - mid;
+            if(diff == citations[mid]){
+                return diff;
+            }else if(diff > citations[mid]){
+                low = mid + 1;
+            }else{
+                high = mid - 1;
+            }
+        }
+
+        return citations.length - low;
+    }
+}

RotateArray.java

@@ -0,0 +1,19 @@
+class Solution {
+    public void rotate(int[] nums, int k) {
+        int n = nums.length;
+        k = k % n;
+        reverse(nums, 0, n - k-1);
+        reverse(nums, n-k, n-1);
+        reverse(nums, 0, n-1);
+    }
+
+    private void reverse(int[] arr, int start, int end){
+        while(start < end){
+            int temp = arr[start];
+            arr[start] = arr[end];
+            arr[end] = temp;
+            start++;
+            end--;
+        }
+    }
+}

TrapRainWater.java

@@ -0,0 +1,18 @@
+class Solution {
+    public int trap(int[] height) {
+        int result = 0;
+        int n = height.length;
+        Stack<Integer> st = new Stack<>();
+        for(int i = 0; i < n; i++){
+            while(!st.isEmpty() && height[i] > height[st.peek()]){
+                int popped = st.pop();
+                if(st.isEmpty()) break;
+                int w = i - st.peek() - 1;
+                int h = Math.min(height[st.peek()], height[i]) - height[popped];
+                result+= h*w;
+            }
+            st.push(i);
+        }
+        return result;
+    }
+}