Merge pull request #111 from chipbk10/Others

Solve Problem 1286 - Iterator for combination

Author: chipbk10

src/others/design/Problem1286_IteratorForCombination.java

@@ -0,0 +1,45 @@
+package others.design;
+
+public class Problem1286_IteratorForCombination {
+
+    class CombinationIterator {
+        String s, cur;
+        int m, n;
+
+        public CombinationIterator(String characters, int combinationLength) {
+            s = characters;
+            m = combinationLength;
+            n = s.length();
+        }
+
+        public String next() {
+            if (cur == null) cur = s.substring(0, m);
+            else {
+                int i = find();
+                int j = binarySearch(0, n-1, cur.charAt(i));
+                String left = (i == 0) ? "" : cur.substring(0, i);
+                String right = s.substring(j+1, j+1+m-i);
+                cur = left+right;
+            }
+            return cur;
+        }
+
+        public boolean hasNext() {
+            return cur == null || find() >= 0;
+        }
+
+        private int find() {
+            for (int i = m-1, j = n-1; i >= 0; i--, j--) {
+                if (cur.charAt(i) != s.charAt(j)) return i;
+            }
+            return -1;
+        }
+
+        private int binarySearch(int lo, int hi, char c) {
+            int mid = lo + (hi-lo)/2;
+            if (s.charAt(mid) > c) return binarySearch(lo, hi-1, c);
+            if (s.charAt(mid) < c) return binarySearch(lo+1, hi, c);
+            return mid;
+        }
+    }
+}