equilibrium index : done

Author: Ram swaroop

src/me/ramswaroop/arrays/EquilibriumIndex.java

@@ -0,0 +1,56 @@
+package me.ramswaroop.arrays;
+
+/**
+ * Created by IntelliJ IDEA.
+ *
+ * @author: ramswaroop
+ * @date: 8/22/15
+ * @time: 11:56 AM
+ */
+public class EquilibriumIndex {
+
+    /**
+     * EQUILIBRIUM INDEX of an array is an index such that the sum of elements at lower
+     * indexes is equal to the sum of elements at higher indexes.
+     * <p/>
+     * For example, in an array A = {-7, 1, 5, 2, -4, 3, 0}
+     * <p/>
+     * 3 is an equilibrium index, because
+     * A[0] + A[1] + A[2] = A[4] + A[5] + A[6]
+     * <p/>
+     * 6 is also an equilibrium index, because sum of zero elements is zero,
+     * i.e., A[0] + A[1] + A[2] + A[3] + A[4] + A[5] = 0
+     * <p/>
+     * 7 is not an equilibrium index, because it is not a valid index of array A.
+     *
+     * @param a
+     * @return equilibrium index of array {@param a}.
+     */
+    public static int getEquilibriumIndex(int[] a) {
+        int totalSum = 0, leftSum = 0;
+
+        for (int i = 0; i < a.length; i++) {
+            totalSum += a[i];
+        }
+
+        for (int i = 0; i < a.length; i++) {
+            totalSum -= a[i]; // totalSum now holds the right sum
+            if (leftSum == totalSum) {
+                return i; // left sum == right sum
+            }
+            leftSum += a[i]; // update left sum
+        }
+
+        return -1;
+    }
+
+    public static void main(String a[]) {
+        System.out.println(getEquilibriumIndex(new int[]{-7, 1, 5, 2, -4, 3, 0}));
+        System.out.println(getEquilibriumIndex(new int[]{-7, 1, 5, 0, 0, 0, 0, 1, 2, -4, 1, 3, 0}));
+        System.out.println(getEquilibriumIndex(new int[]{4, 5, 2, 1, 6, 7, 8, 0, 1}));
+        System.out.println(getEquilibriumIndex(new int[]{0}));
+        System.out.println(getEquilibriumIndex(new int[]{0, 0, 0}));
+        System.out.println(getEquilibriumIndex(new int[]{1, 1}));
+        System.out.println(getEquilibriumIndex(new int[]{1, 1, 1}));
+    }
+}