4Sum

	public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
		if (num == null || num.length < 4)
			return new ArrayList<ArrayList<Integer>>();
		Arrays.sort(num);

		return genericSumSorted(num, target, 0, num.length, 4);
	}

	public ArrayList<ArrayList<Integer>> genericSumSorted(int[] num,
			int target, int offSet, int len, int numToSum) {
		int loopMax = len - numToSum + 1;
		ArrayList<ArrayList<Integer>> rv = new ArrayList<ArrayList<Integer>>();

		if (numToSum == 1) {
			for (int i = offSet; i < loopMax; i++) {
				if (num[i] == target) {
					ArrayList<Integer> newSet = new ArrayList<Integer>();
					newSet.add(num[i]);
					rv.add(newSet);
					break;
				}
			}
		} else {
			int lastValue = num[offSet] - 1;
			for (int i = offSet; i < loopMax; i++) {
				if (num[i] == lastValue)
					continue;
				lastValue = num[i];
				int newTarget = target - num[i];

				ArrayList<ArrayList<Integer>> sub = genericSumSorted(num,
						newTarget, i + 1, len, numToSum - 1);
				for (ArrayList<Integer> subDetail : sub) {
					subDetail.add(0, lastValue);
					rv.add(subDetail);
				}
			}
		}
		return rv;
	}

3Sum Closest

	public int threeSumClosest(int[] num, int target) {
		if (num == null)
			return target;

		int len = num.length;
		if (len < 3) {
			int sum = 0;
			for (int i = 0; i < len; i++)
				sum += num[i];
			return sum;
		}

		Arrays.sort(num);
		return findClosestSumSorted(num, target, 0, len, 3);
	}

	public int findClosestSumSorted(int[] num, int target, int offSet, int len,
			int numToSum) {
		int loopMax = len - numToSum + 1;
		if (numToSum == 1) {
			int prevBestValue = num[offSet];
			int prevDiff = Math.abs(target - num[offSet]);
			for (int i = offSet + 1; i < loopMax; i++) {
				int currentDiff = Math.abs(target - num[i]);
				if (currentDiff < prevDiff) {
					prevBestValue = num[i];
					prevDiff = currentDiff;
				} else if (currentDiff > prevDiff)
					break;
			}
			return prevBestValue;
		} else {
			int prevBestValue = 0, prevDiff = Integer.MAX_VALUE;
			int lastValue = num[offSet] - 1;
			for (int i = offSet; i < loopMax; i++) {
				if (lastValue == num[i])
					continue;

				lastValue = num[i];
				int newTarget = target - num[i];
				int findValue = findClosestSumSorted(num, newTarget, i + 1,
						len, numToSum - 1);
				int currentDiff = Math.abs(newTarget - findValue);
				if (currentDiff < prevDiff) {
					prevBestValue = num[i] + findValue;
					prevDiff = currentDiff;
				} else if (currentDiff > prevDiff)
					break;
			}
			return prevBestValue;
		}
	}

3Sum

	public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
		if (num == null || num.length < 3)
			return new ArrayList<ArrayList<Integer>>();
		Arrays.sort(num);
		return genericSumSorted(num, 0, 0, num.length, 3);
	}

	public ArrayList<ArrayList<Integer>> genericSumSorted(int[] num,
			int target, int offSet, int len, int numToSum) {
		int loopMax = len - numToSum + 1;
		ArrayList<ArrayList<Integer>> rv = new ArrayList<ArrayList<Integer>>();

		if (numToSum == 1) {
			for (int i = offSet; i < loopMax; i++) {
				if (num[i] == target) {
					ArrayList<Integer> newSet = new ArrayList<Integer>();
					newSet.add(num[i]);
					rv.add(newSet);
					break;
				}
			}
		} else {
			int lastValue = num[offSet] - 1;
			for (int i = offSet; i < loopMax; i++) {
				if (num[i] == lastValue)
					continue;
				lastValue = num[i];
				int newTarget = target - num[i];

				ArrayList<ArrayList<Integer>> sub = genericSumSorted(num,
						newTarget, i + 1, len, numToSum - 1);
				for (ArrayList<Integer> subDetail : sub) {
					subDetail.add(0, lastValue);
					rv.add(subDetail);
				}
			}
		}
		return rv;
	}

Longest Common Prefix

 public String longestCommonPrefix(String[] strs) {
  if (strs == null)
   return null;

  int len = strs.length;
  if (len == 0)
   return "";

  int shortStrIdx = 0;
  int shortStrLen = strs[0].length();
  for (int i = 1; i < len; i++) {
   int strLen = strs[i].length();
   if (strLen < shortStrLen) {
    shortStrIdx = i;
    shortStrLen = strLen;
   } else if (strLen == shortStrLen) {
    for (int j = 0; j < strLen; j++) {
     if (strs[shortStrIdx].charAt(j) != strs[i].charAt(j)) {
      shortStrLen = j;
      break;
     }
    }
    if (shortStrLen == 0)
     return "";
   }
  }

  for (int i = 0; i < len; i++) {
   if (i == shortStrIdx)
    continue;

   for (int j = 0; j < shortStrLen; j++) {
    if (strs[shortStrIdx].charAt(j) != strs[i].charAt(j)) {
     shortStrLen = j;
     break;
    }
    if (shortStrLen == 0)
     return "";
   }
  }

  return strs[shortStrIdx].substring(0, shortStrLen);
 }