Minimum Size Subarray Sum 解答

Question

Given an array of n positive integers and a positive integer s, find the minimal length of a subarray of which the sum ≥ s. If there isn't one, return 0 instead.

For example, given the array [2,3,1,2,4,3] and s = 7,
the subarray [4,3] has the minimal length under the problem constraint.

Solution 1 -- Two Pointers

Following the thought of slide window, we apply two pointers here to solve the problem, one for left and one for right.

If current sum >= target, left++;

If current sum < target, right--;

Remember to check right index before getting nums[right], and sum value is nums[0] at beginning.

Time complexity O(n).

public class Solution {
    public int minSubArrayLen(int s, int[] nums) {
        if (nums == null || nums.length == 0)
            return 0;
        int length = nums.length, left = 0, right = 0, result = length, sum = nums[0];
        while (right < length) {
            if (left == right) {
                if (nums[left] >= s) {
                    return 1;
                } else {
                    right++;
                    if (right < length)
                        sum += nums[right];
                    else
                        break;
                }
            } else {
                if (sum < s) {
                    right++;
                    if (right < length)
                        sum += nums[right];
                    else
                        break;
                } else {
                    result = Math.min(result, right - left + 1);
                    sum -= nums[left];
                    left++;
                }
            }
        }
        if (left == 0 && sum < s)
            result = 0;
        return result;
    }
}