README_EN.md

1814. Count Nice Pairs in an Array

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Description

You are given an array nums that consists of non-negative integers. Let us define rev(x) as the reverse of the non-negative integer x. For example, rev(123) = 321, and rev(120) = 21. A pair of indices (i, j) is nice if it satisfies all of the following conditions:

• 0 <= i < j < nums.length
• nums[i] + rev(nums[j]) == nums[j] + rev(nums[i])

Return the number of nice pairs of indices. Since that number can be too large, return it modulo 109 + 7.

 

Example 1:

Input: nums = [42,11,1,97]
Output: 2
Explanation: The two pairs are:
 - (0,3) : 42 + rev(97) = 42 + 79 = 121, 97 + rev(42) = 97 + 24 = 121.
 - (1,2) : 11 + rev(1) = 11 + 1 = 12, 1 + rev(11) = 1 + 11 = 12.

Example 2:

Input: nums = [13,10,35,24,76]
Output: 4

 

Constraints:

• 1 <= nums.length <= 105
• 0 <= nums[i] <= 109

Solutions

Python3

class Solution:
    def countNicePairs(self, nums: List[int]) -> int:
        def rev(x):
            y = 0
            while x:
                y = y * 10 + x % 10
                x //= 10
            return y

        cnt = Counter(x - rev(x) for x in nums)
        mod = 10**9 + 7
        return sum(v * (v - 1) // 2 for v in cnt.values()) % mod

class Solution:
    def countNicePairs(self, nums: List[int]) -> int:
        def rev(x):
            y = 0
            while x:
                y = y * 10 + x % 10
                x //= 10
            return y

        ans = 0
        mod = 10**9 + 7
        cnt = Counter()
        for x in nums:
            y = x - rev(x)
            ans += cnt[y]
            cnt[y] += 1
        return ans % mod

Java

class Solution {
    public int countNicePairs(int[] nums) {
        Map<Integer, Integer> cnt = new HashMap<>();
        for (int x : nums) {
            int y = x - rev(x);
            cnt.merge(y, 1, Integer::sum);
        }
        final int mod = (int) 1e9 + 7;
        long ans = 0;
        for (int v : cnt.values()) {
            ans = (ans + (long) v * (v - 1) / 2) % mod;
        }
        return (int) ans;
    }

    private int rev(int x) {
        int y = 0;
        for (; x > 0; x /= 10) {
            y = y * 10 + x % 10;
        }
        return y;
    }
}

class Solution {
    public int countNicePairs(int[] nums) {
        Map<Integer, Integer> cnt = new HashMap<>();
        final int mod = (int) 1e9 + 7;
        int ans = 0;
        for (int x : nums) {
            int y = x - rev(x);
            ans = (ans + cnt.getOrDefault(y, 0)) % mod;
            cnt.merge(y, 1, Integer::sum);
        }
        return ans;
    }

    private int rev(int x) {
        int y = 0;
        for (; x > 0; x /= 10) {
            y = y * 10 + x % 10;
        }
        return y;
    }
}

C++

class Solution {
public:
    int countNicePairs(vector<int>& nums) {
        auto rev = [](int x) {
            int y = 0;
            for (; x > 0; x /= 10) {
                y = y * 10 + x % 10;
            }
            return y;
        };
        unordered_map<int, int> cnt;
        for (int& x : nums) {
            int y = x - rev(x);
            cnt[y]++;
        }
        long long ans = 0;
        const int mod = 1e9 + 7;
        for (auto& [_, v] : cnt) {
            ans = (ans + 1ll * v * (v - 1) / 2) % mod;
        }
        return ans;
    }
};

class Solution {
public:
    int countNicePairs(vector<int>& nums) {
        auto rev = [](int x) {
            int y = 0;
            for (; x > 0; x /= 10) {
                y = y * 10 + x % 10;
            }
            return y;
        };
        unordered_map<int, int> cnt;
        int ans = 0;
        const int mod = 1e9 + 7;
        for (int& x : nums) {
            int y = x - rev(x);
            ans = (ans + cnt[y]++) % mod;
        }
        return ans;
    }
};

Go

func countNicePairs(nums []int) (ans int) {
	rev := func(x int) (y int) {
		for ; x > 0; x /= 10 {
			y = y*10 + x%10
		}
		return
	}
	cnt := map[int]int{}
	for _, x := range nums {
		y := x - rev(x)
		cnt[y]++
	}
	const mod int = 1e9 + 7
	for _, v := range cnt {
		ans = (ans + v*(v-1)/2) % mod
	}
	return
}

func countNicePairs(nums []int) (ans int) {
	rev := func(x int) (y int) {
		for ; x > 0; x /= 10 {
			y = y*10 + x%10
		}
		return
	}
	cnt := map[int]int{}
	const mod int = 1e9 + 7
	for _, x := range nums {
		y := x - rev(x)
		ans = (ans + cnt[y]) % mod
		cnt[y]++
	}
	return
}

JavaScript

/**
 * @param {number[]} nums
 * @return {number}
 */
var countNicePairs = function (nums) {
    const rev = x => {
        let y = 0;
        for (; x > 0; x = Math.floor(x / 10)) {
            y = y * 10 + (x % 10);
        }
        return y;
    };
    const cnt = new Map();
    for (const x of nums) {
        const y = x - rev(x);
        cnt.set(y, (cnt.get(y) | 0) + 1);
    }
    let ans = 0;
    const mod = 1e9 + 7;
    for (const [_, v] of cnt) {
        ans = (ans + Math.floor((v * (v - 1)) / 2)) % mod;
    }
    return ans;
};

/**
 * @param {number[]} nums
 * @return {number}
 */
var countNicePairs = function (nums) {
    const rev = x => {
        let y = 0;
        for (; x > 0; x = Math.floor(x / 10)) {
            y = y * 10 + (x % 10);
        }
        return y;
    };
    let ans = 0;
    const mod = 1e9 + 7;
    const cnt = new Map();
    for (const x of nums) {
        const y = x - rev(x);
        const v = cnt.get(y) | 0;
        ans = (ans + v) % mod;
        cnt.set(y, v + 1);
    }
    return ans;
};

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