# Java one pass O(n) solution with explanation

• diff[i] is "count of 1s" minus "count of 0s" so far.

For given i and j, if diff[i] == diff[j], then the subarray between i and j is a contiguous array as defined in the questions.

For any value of diff[], we save the index of the first item in the hashmap. Then once the value appears again, we get the length of the subarray between the current index and the index of the first item.

``````public int findMaxLength(int[] nums) {
int res = 0;
int n = nums.length;

int[] diff = new int[n + 1];

Map<Integer, Integer> map = new HashMap<>();
map.put(0, 0);

for (int i = 1; i <= n; i++) {
diff[i] = diff[i - 1] + (nums[i - 1] == 0 ? -1 : 1);

if (!map.containsKey(diff[i]))
map.put(diff[i], i);
else
res = Math.max(res, i - map.get(diff[i]));
}

return res;
}``````

• @yuhaowang001 it can be more easier.

``````class Solution {
public:
int findMaxLength(vector<int>& nums) {
map<int, int> m;
m.insert(pair<int, int>(0, 0));
int ans = 0, k = 0;
for (int i = 0; i < nums.size(); i++) {
k = k + (nums[i] == 0 ? -1 : 1);
if (m.find(k) != m.end())
ans = ans < i + 1 - m[k] ? i + 1 - m[k] : ans;
else
m[k] = i + 1;
}
return ans;
}
};
``````

• @yuhaowang001
Thanks for sharing the solution.
I do not understand the rationale behind why does subtracting the previous index from the current index for the same value give us the correct solution?

• Let me add some explanation.
If we keep track of the total count of ones and zeros from `0` to `len - 1`, then to valid whether the subarray from `i` to `j` is valid, we just have to check if `ones[j] - ones[i - 1] == zeros[j] - zeros[i - 1]`.
Naively it will take O(N^2) to compute. But that equation is the same as `ones[j] - zeros[j] == one[i - 1] - zeros[i - 1]`, and it is `diff[j] == diff[i - 1]`, then we can use hashmap to reduce the time complexity to O(N).
Here is my implementation:

``````public class Solution {
public int findMaxLength(int[] nums) {
HashMap<Integer, Integer> diff2idx = new HashMap<>();
int res = 0;
int diff = 0;
diff2idx.put(0, -1);
int ones = 0;
for (int i = 0; i < nums.length; i++) {
ones += nums[i];
diff = ones - (i + 1 - ones); // ones - zeros
if (diff2idx.containsKey(diff)) {
res = Math.max(res, i - diff2idx.get(diff));
} else {
diff2idx.put(diff, i);
}
}
return res;
}
}
``````

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