LeetCode brush title: Forty title rainwater

Given  n  nonnegative integer width of each column of a height map calculated Click column arrangement, it can take much rain after rain.

The above is an array [0,1,0,2,1,0,1,3,2,1,2,1] FIG highly represented, in this case, can take six units rainwater (blue portion represents the rain). Thank Marcos contribution to this figure.

Example:

Input: [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6

int trap(int* height, int heightSize) {
    int ans = 0, top = -1;
    int *stack = (int *)malloc(sizeof(int) * heightSize);
    for (int i = 0; i < heightSize; i++) {
        while (top != -1 && height[stack[top]] < height[i]) {
            int w1 = (top == 0 ? 0 : height[stack[top - 1]] - height[stack[top]]);
            int w2 = height[i] - height[stack[top]];
            int l = (top == 0 ? 0 : i - stack[top - 1] - 1);
            years + = l * f min (w1, w2);
            top--;
        }
        stack[++top] = i;
    }
    free(stack);
    return ans;
}