560. Subarray Sum Equals K
Given an array of integers and an integer k,
you need to find the total number of continuous subarrays whose sum equals to k. Example 1: Input:nums = [1,1,1], k = 2 Output: 2 Note: The length of the array is in range [1, 20,000]. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].
fb: 只用返回true or false。第二题先用set做,后来让改用constant space, 就用了sliding window这样
subarray sum 问题常用hashmap, 存count 值和坐标, 动归的感觉啊
fb:问了数组包含/不包含负数两种情况,
public int subarraySum(int[] nums, int k) { int sum = 0, result = 0; Map<Integer, Integer> preSum = new HashMap<>(); preSum.put(0, 1); for (int i = 0; i < nums.length; i++) { sum += nums[i]; if (preSum.containsKey(sum - k)) { result += preSum.get(sum - k); } // 当加着加着出现两个一样的sum时, 要在他的value上加1, 因为可以有多个连续的串 preSum.put(sum, preSum.getOrDefault(sum, 0) + 1); } return result; }
要用 preSum.put(0, 1); 是得result 加的值可以来自map中的多个.
不能 if (sum == k) {
result++;
}
因为:
Input:[0,0,0,0,0,0,0,0,0,0] 0
Output:10
Expected:55
对比523. Continuous Subarray Sum, 0 的indice放-1, 0的count 放1;
public boolean checkSubarraySum(int[] nums, int k) { Map<Integer, Integer> map = new HashMap<Integer, Integer>(){{put(0,-1);}};; int runningSum = 0; for (int i=0;i<nums.length;i++) { runningSum += nums[i]; if (k != 0) runningSum %= k; Integer prev = map.get(runningSum); if (prev != null) { if (i - prev > 1) return true; } else map.put(runningSum, i); } return false; }
对比subarray sum
public class Solution { public boolean isIsomorphic(String s, String t) { if (s==null || t==null || s.length() != t.length()) return false; HashMap<Character, Character> map = new HashMap<Character, Character>(); for (int i=0; i<s.length(); i++) { if (!map.containsKey(s.charAt(i))) { if (!(map.values().contains(t.charAt(i)))) { map.put(s.charAt(i), t.charAt(i)); } else return false; } else if (map.get(s.charAt(i)) != t.charAt(i)) return false; } return true; } }