剑指offer专项突击版 --- 第 4 天

class Solution {
    
    public int subarraySum(int[] nums, int k) {
    
        Map<Integer,Integer> map = new HashMap<>();
        map.put(0, 1);
        int sum = 0;
        int count = 0;
        for (int i=0;i<nums.length;++i) {
    
            sum += nums[i];
            count += map.getOrDefault(sum-k, 0);
            map.put(sum, map.getOrDefault(sum, 0) + 1);
        }
        return count;
    }
}

class Solution {
    
    public int findMaxLength(int[] nums) {
    
        Map<Integer,Integer> map = new HashMap<>();
        map.put(0,-1);
        int sum = 0;
        int ret = 0;
        for(int i = 0; i < nums.length; i++){
    
            sum += nums[i] == 0 ? -1 : 1;
            if(map.containsKey(sum)){
    
                ret = Math.max(ret, i - map.get(sum));
            }else{
    
                map.put(sum,i);
            }
        }
        return ret;
    }
}

class Solution {
    
    public int pivotIndex(int[] nums) {
    
        for(int i =0; i < nums.length; i++){
    
            int num = 0;
            for(int j = 0; j < nums.length; j++){
    
                if(j < i){
    
                    num += nums[j];
                }else if(j == i){
    
                    continue;
                }else{
    
                    num -= nums[j];
                }
            }
            if(num == 0){
    
                return i;
            }
        }
        return -1;
    }
}

class NumMatrix {
    
    int[][] sums;

    public NumMatrix(int[][] matrix) {
    
        int m = matrix.length;
        if (m > 0) {
    
            int n = matrix[0].length;
            sums = new int[m + 1][n + 1];
            for (int i = 0; i < m; i++) {
    
                for (int j = 0; j < n; j++) {
    
                    sums[i + 1][j + 1] = sums[i][j + 1] + sums[i + 1][j] - sums[i][j] + matrix[i][j];
                }
            }
        }
    }
    
    public int sumRegion(int row1, int col1, int row2, int col2) {
    
        return sums[row2 + 1][col2 + 1] - sums[row1][col2 + 1] - sums[row2 + 1][col1] + sums[row1][col1];
    }
}

/**
 * Your NumMatrix object will be instantiated and called as such:
 * NumMatrix obj = new NumMatrix(matrix);
 * int param_1 = obj.sumRegion(row1,col1,row2,col2);
 */