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public class Solution {
    public int maxSubArrayLen(int[] nums, int k) {
        if (nums == null || nums.length < 1) return 0;
        int len = nums.length;
        int[] prefixSum = new int[len + 1];
        for (int i = 1; i <= len; i++) {
            prefixSum[i] = nums[i - 1] + prefixSum[i - 1];
        }
        Map<Integer, Integer> map = new HashMap<>();
        map.put(0, 0);
        int res = 0;
        for (int i = 1; i <= len; i++) {
            if (map.containsKey(prefixSum[i] - k)) {
                res = Math.max(res, i - map.get(prefixSum[i] - k));
            }
            if (!map.containsKey(prefixSum[i])) // keep only 1st duplicate as we want first index as left as possible
                map.put(prefixSum[i], i);
        }
        return res;
    }
}